Parity: engine now accepts flat-form pack-namespaced component keys — assembler classifiers don't #652

labelle-toolkit/labelle-engine#806 widened the engine's flat-form key classification: <prefix>__<Pascal> keys at entity scope are now real component keys (previously silently dropped — the FP "flat pack component won't attach" trap). Assembler-side classifiers still treat only PascalCase (+@) as flat content:

• scene_manifest.isAllowedTopLevelKey rejects a flat namespaced key at a file's TOP level (UnknownSceneKey) — now a false rejection of a shape the engine accepts;
• hasFlatEntityShapeKey / checkHybridForm don't count namespaced keys, so namespaced-flat + wrapper mixes aren't flagged as hybrid while the engine warns wrapper-wins-and-drops;
• scene_name_lint.collectComponentRefs misses flat namespaced refs at entity scope (.entity => isPascalCase);
• pack_refs pass 1 wrapper migration doesn't move flat namespaced pairs (mostly moot for packs — pass 2 rewrites bare names before they're namespaced — but game-root files can author them directly).

Fix shape: share one isComponentKeyShape twin (byte-parity with engine unified_format) across these sites, mirroring how isTargetKey was propagated in #650.

Refs: labelle-toolkit/labelle-engine#806, #650, #651.