Govern task context and advance execution pointer

docs/status/M16-GAP-00020.md

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+# M16-GAP-00020 Status
+
+status: done
+task: Scene data-block LOCAL_EXACT slice
+updated: 2026-08-19 America/New_York
+
+scope: Blender 5.2 Scene identity and bounded timeline/render/unit metadata: current/start/end frames, FPS, render engine and unit scale.
+
+evidence:
+
+- `tools/web/generated/M16-GAP-00020.py` generated `WebGapScene` with frame 48 in range 12-180, 30 FPS, Eevee render and metric unit scale 0.25.
+- Desktop report reopened after save and matched exactly for scene identity, timeline, render engine and unit metadata.
+- WASM/Main opened the same fixture, matched the Scene record and global frame/coordinate metadata, and save/reopen matched the complete scene and frame snapshots exactly.
+- A truncated malformed Blend submitted to an already loaded Main was rejected and the original snapshot remained unchanged.
+- `build_blender_5.2.0/bin/blender -b --factory-startup --python tools/web/generated/M16-GAP-00020.py -- tests/files/web/generated/M16-GAP-00020-datablock-Scene.blend` exited 0.
+- `npm --prefix web run test:generated-gap -- --task M16-GAP-00020` exited 0.
+- `node tools/web/check-generated-gap.mjs --task M16-GAP-00020` exited 0.
+
+artifacts: `tests/golden/M16-GAP-00020/manifest.json` contains SHA-256 for the parent manifest, fixture, generator, desktop/Web checkers, reader/protocol/runtime and both reports.
+
+knownRisk: This closes one `datablock:Scene` metadata slice only. Scene sequencing, compositor, render execution and all other generated gaps remain separate tasks.
+
+rollback: remove this task's fixture, generator, checker branch, reports, manifest, status and context; restore `M16-GAP-00019` as the queue tail without editing its successful evidence.
+
+nextTask: `M16-GAP-00021`