bim.store cost/structural refs return a raw rather than normalized modelId

[louistrue]

Split out of review on #5231, where it was flagged but is pre-existing and affects the shipped cost path, so it did not belong in that PR.

createStoreAdapter's per-call resolution returns the caller's raw model id:

return { modelId: requested, store: dataStore, editor, mutationView, ownerHistoryId };

while addEntity and buildElement return normalizeMutationModelId(...) — __legacy__ for a legacy public id. entityRefToString serializes modelId unchanged, so the same entity can receive different serialized keys depending on which store method created its ref.

This is verbatim from main before #5231 (git show <pre-5231>:apps/viewer/src/sdk/adapters/store-adapter.ts, the createCostStoreBackend closure); #5231 only extracted that closure so cost and structural share one resolver, which is what made the inconsistency visible.

Fixing it changes the ref identity bim.store.addCost* hands back today, so it wants its own change where it can be reviewed against the cost tests and any consumer that keys off EntityRef.modelId — not a drive-by in a structural PR.

Suggested fix: return normalized from the shared resolver, and check store-adapter-cost-undo.test.ts plus anything comparing entityRefToString output.