refactor(M03): extract M2 mesh resource traversal

Work-Package: M03-RND-M2-MESH-RESOURCE-EXTRACTOR-001 Agent: sindo-main-codex Tests: 36 headless renderer/coordinate contracts pass; checkpoint dry-run 7/7; documentation and coordination gates pass Fidelity: preserves first-Mesh preorder and temporary PackedScene cleanup; no visual parity claim
2026-07-17 13:02:40 +04:00
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+# M2 Mesh Resource Extractor
+
+## Metadata
+
+| Field | Value |
+|---|---|
+| Status | Implemented extraction |
+| Target/work package | M03 / `M03-RND-M2-MESH-RESOURCE-EXTRACTOR-001` |
+| Owners | First-Mesh selection and temporary PackedScene instance lifetime |
+| Last verified | Worktree `work/sindo-main-codex/m03-m2-mesh-resource-extractor`, 2026-07-17 |
+| Profiles/capabilities | Static M2 threaded cache and synchronous raw/prototype fallback paths |
+
+## Purpose
+
+Select the first M2 Mesh from a direct Resource, PackedScene or existing Node
+subtree while keeping traversal and temporary-node lifetime outside
+`StreamingWorldLoader`.
+
+## Non-goals
+
+- Request/load Resources or choose cache paths.
+- Load raw M2 data, refresh materials or rebuild Meshes.
+- Own Mesh/prototype/missing caches or pending/finalize queues.
+- Create MultiMeshes, attach Nodes or consume render permits.
+- Change first-Mesh selection order or visible output.
+
+## Context and boundaries
+
+```mermaid
+flowchart LR
+    Pipeline[M2 load pipeline] --> Loader[StreamingWorldLoader]
+    Loader --> Resource[Loaded Mesh or PackedScene Resource]
+    Resource --> Extractor[M2MeshResourceExtractor]
+    Extractor -->|first Mesh or null| Loader
+    Loader --> Prepare[Material refresh/rebuild]
+    Prepare --> Cache[M2MeshResourceCacheState]
+    Prototype[Existing M2 Node subtree] --> Extractor
+```
+
+The extractor may inspect `Resource`, `PackedScene`, `Node`, `MeshInstance3D`
+and `Mesh` engine types. It has no ResourceLoader, filesystem, worker, cache,
+scheduler, builder, gameplay, network or editor dependency.
+
+## Public API
+
+| Symbol | Kind | Purpose | Thread/lifetime | Errors |
+|---|---|---|---|---|
+| `extract_first_mesh(resource)` | Query with temporary ownership | Return direct Mesh or first Mesh from one PackedScene instance | Renderer main thread; temporary root freed before return | Null/unsupported/empty returns null |
+| `find_first_mesh_in_subtree(root)` | Query | Return first Mesh in historical depth-first preorder | Renderer main thread; caller-owned subtree unchanged | Null/no Mesh returns null |
+
+## Inputs and outputs
+
+| Direction | Contract/data | Producer | Consumer | Ownership | Thread/lifetime |
+|---|---|---|---|---|---|
+| Input | Direct Mesh Resource | Loader threaded finalizer | Extractor | Borrowed Resource | One call |
+| Input | PackedScene Resource | Loader threaded finalizer | Extractor | Borrowed scene; extractor owns temporary instance | Temporary root until return |
+| Input | Existing prototype/rebuild Node subtree | Loader builder/fallback | Extractor | Borrowed Node tree | One traversal |
+| Output | First Mesh reference | Extractor | Loader preparation/cache adapter | Borrowed exact reference | Cache may retain later |
+
+The only side effect is synchronous creation and immediate `free()` of a
+temporary PackedScene instance.
+
+## Data flow
+
+```mermaid
+flowchart TD
+    Input[Resource or Node subtree] --> Direct{Direct Mesh?}
+    Direct -->|yes| Return[Return exact Mesh]
+    Direct -->|no| Packed{PackedScene?}
+    Packed -->|yes| Instantiate[Instantiate temporary root]
+    Packed -->|no| Traverse[Traverse caller-owned subtree]
+    Instantiate --> TraverseTemp[Depth-first preorder traversal]
+    TraverseTemp --> Free[Free temporary root]
+    Free --> ReturnOrNull[Return first Mesh or null]
+    Traverse --> ReturnOrNull
+```
+
+## Lifecycle/state
+
+```mermaid
+stateDiagram-v2
+    [*] --> Stateless
+    Stateless --> TemporaryInstance: extract PackedScene
+    TemporaryInstance --> Traversing: instantiate succeeded
+    Traversing --> Stateless: free temporary root before return
+    Stateless --> Stateless: direct Mesh/subtree/invalid input
+```
+
+No state or Resource reference is retained between calls.
+
+## Main sequence
+
+```mermaid
+sequenceDiagram
+    participant Loader as StreamingWorldLoader
+    participant Extractor as M2MeshResourceExtractor
+    participant Scene as PackedScene
+    Loader->>Extractor: extract_first_mesh(loaded Resource)
+    alt direct Mesh
+        Extractor-->>Loader: same Mesh reference
+    else PackedScene
+        Extractor->>Scene: instantiate()
+        Scene-->>Extractor: temporary root
+        Extractor->>Extractor: depth-first first-Mesh search
+        Extractor->>Extractor: temporary_root.free()
+        Extractor-->>Loader: Mesh or null
+    end
+    Loader->>Loader: prepare and cache Mesh or mark missing
+```
+
+## Dependency diagram
+
+```mermaid
+flowchart TB
+    Loader[StreamingWorldLoader] --> Extractor[M2MeshResourceExtractor]
+    Extractor --> Types[Resource / PackedScene / Node / Mesh]
+    Loader --> Pipeline[M2MeshLoadPipelineState]
+    Loader --> Cache[M2MeshResourceCacheState]
+    Loader --> Classifier[M2RuntimeMeshRebuildClassifier]
+    Extractor -. no dependency .-> Pipeline
+    Extractor -. no dependency .-> Cache
+    Extractor -. no dependency .-> Classifier
+```
+
+## Ownership, threading and resources
+
+- The renderer main thread performs all traversal and PackedScene instantiation.
+- Direct and subtree Mesh references are borrowed without duplication.
+- A PackedScene temporary root is extractor-owned and synchronously freed.
+- Existing prototype/rebuild subtrees remain caller-owned and are never mutated.
+- Cache retention and Mesh material preparation remain loader/service concerns.
+
+## Errors, cancellation and recovery
+
+| Failure/state | Detection | Behavior | Diagnostic | Recovery |
+|---|---|---|---|---|
+| Null/unsupported Resource | Type guard | Return null | Contract verifier | Loader marks missing/falls back |
+| PackedScene instantiate failure | Null root | Return null | Existing loader behavior | Rebuild cache/source |
+| No Mesh in subtree | Traversal exhaustion | Return null | Contract verifier | Loader marks missing/falls back |
+| Null MeshInstance Mesh | Null traversal result | Parent continues with later siblings | Order verifier | Normal sibling search |
+| Shutdown/cancel | No retained state | No action | Ownership docs | Caller owns surrounding job |
+
+## Configuration and capabilities
+
+The module adds no settings or profile branches. Cache format order, material
+refresh version, render permits and batching remain unchanged outside it.
+
+## Persistence, cache and migration
+
+The extractor is stateless and serializes nothing. Existing `.tscn`/`.glb`
+formats, cache versions and rebuild metadata are unchanged; no migration or
+rebake is required.
+
+## Diagnostics and observability
+
+The module emits no logs and retains no counters. Loader correlation remains the
+normalized M2 path. Its verifier measures traversal time and temporary Node count.
+
+## Verification
+
+- `verify_m2_mesh_resource_extractor.gd`: direct/invalid Resources, depth-first
+  child order, null-Mesh sibling continuation, subtree ownership, PackedScene
+  exact Mesh identity, temporary Node release, source boundaries and 10,000
+  bounded traversals.
+- M2 pipeline/cache/classifier/material/shutdown/build regressions cover adjacent
+  behavior. No asset-backed visual parity claim is made.
+
+## Extension points
+
+- Material refresh/rebuild can become a separate finalizer consuming the Mesh
+  returned here and storing its result in `M2MeshResourceCacheState`.
+- Broader generic scene traversal is intentionally excluded until another real
+  consumer requires the same exact contract.
+
+## Capability status
+
+| Capability | Status | Evidence | Gap/next step |
+|---|---|---|---|
+| Direct/PackedScene first-Mesh extraction | Implemented extraction | Resource/order/lifetime verifier | Asset-backed corrupt-scene fixture pending |
+| Prototype/rebuild subtree selection | Implemented extraction | Synthetic preorder verifier | Asset-backed M2 traversal pending |
+| Material refresh/rebuild finalization | Existing loader-owned | Classifier/material regressions | Separate finalizer extraction next |
+
+## Known gaps and risks
+
+- PackedScene instantiation remains synchronous main-thread work behind the
+  existing finalize permit.
+- The first Mesh rule intentionally ignores additional MeshInstances.
+- No proprietary M2 corpus, corrupt cache, leak/descriptor-pressure, p95/p99 or
+  paired original-client capture is included.
+
+## Source map
+
+| Path | Responsibility |
+|---|---|
+| `src/render/m2/m2_mesh_resource_extractor.gd` | First-Mesh traversal and temporary PackedScene lifetime |
+| `src/scenes/streaming/streaming_world_loader.gd` | ResourceLoader, preparation, caching, missing state and materialization |
+| `src/render/m2/m2_mesh_resource_cache_state.gd` | Prepared Mesh retention |
+| `src/tools/verify_m2_mesh_resource_extractor.gd` | Resource/order/lifetime/boundary/timing regression |
+
+## Related decisions and references
+
+- [`m2-mesh-resource-cache-state.md`](m2-mesh-resource-cache-state.md)
+- [`m2-mesh-load-pipeline-state.md`](m2-mesh-load-pipeline-state.md)
+- [`m2-runtime-mesh-rebuild-classifier.md`](m2-runtime-mesh-rebuild-classifier.md)
+- [`world-renderer.md`](world-renderer.md)
+- [`../../RENDER.md`](../../RENDER.md)
+- [`../../targets/roadmap/02-rendering-and-graphics.md`](../../targets/roadmap/02-rendering-and-graphics.md)