Advance M16 modifier parity and context governance

docs/status/M16-GAP-00087.md

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+# M16-GAP-00087 Status
+
+status: done
+task: PARTICLE_INSTANCE modifier LOCAL_EXACT slice
+updated: 2026-08-20 America/New_York
+
+scope: Mesh PARTICLE_INSTANCE modifier stack exposure for `mesh:WebGapParticleInstanceMesh`.
+
+evidence:
+
+- Desktop fixture exposes `WebGapParticleInstance` as PARTICLE_INSTANCE type code 20; save/reopen is exact.
+- WASM/Main exposes PARTICLE_INSTANCE type code 20 and base metadata parameters on the mesh scene node.
+- Malformed Blend input is rejected without mutating Main.
+- Generator, npm comparator, and direct comparator exited 0.
+
+knownRisk: Particle source binding and instance evaluation remain unavailable; this slice covers modifier stack transport and base metadata.
+
+nextTask: `M16-GAP-00088`