fix: harden type-safe .riv schema system (review findings)

[mfazekas]

Stacked on #352 (git-town: main → feat/type-safe-riv-schema-2 → this). Addresses the findings from a fresh review of the schema system; every finding was reproduced with compile probes before fixing, and each fix is pinned by new tsd tests.

Type level (useViewModelInstance, TypedViewModelInstance):

1. Untyped files no longer dead-end. Everything from RiveFileFactory.* / useRiveFile(require(...)) carries the base RiveFileSchema, and the old typed overload turned their property paths into never while __vmBrand blocked the untyped hooks — existing code that compiles on main failed to compile. All name/path constraints now degrade to string when the schema isn't statically known.
2. viewModelName actually constrains: a typo on a schema-typed file is a hard error instead of silently falling through to the untyped overload (which disabled all downstream path checking).
3. required: true keeps its non-null narrowing on typed calls, and the sync (non-async) path stays @deprecated for typed calls too.
4. Nested paths: instance accessors and useRiveEnum now accept them like the sibling hooks, up to two hops ('Coin/Property_Of_Item/Item_Selection'), with enum value unions resolved through the path.
5. List elements are plain ViewModelInstance — the previous all-VMs union made every accessor parameter intersect to never.

Generator/CI:

6. rive-gen-types --all was silently validating only a prefix of the assets: image decode without WebGL stalls the WASM load() forever, the pending promise drains bun's event loop, and the process exits 0 mid-batch. The loader now claims assets without decoding, each load() has a 30s watchdog, and the batch continues past failures and exits non-zero. click-count.riv.d.ts is new in this PR — it sat in the skipped tail, so its schema had never been generated and CI didn't notice.
7. Schemas with no ViewModels emit viewModels: {} — omitting the key failed the RiveFileSchema constraint and silently degraded those assets (7 of them) to fully untyped.
8. Names are escaped in emitted .d.ts files; enum values containing | fall back to untyped enum with a warning; the header uses a working /* eslint-disable */; the CI sync check also catches untracked .riv.d.ts files.