test: expand coverage for build cache, CLI, watch modes, and parity

[rjrodger]

Summary

A test-coverage pass surfaced several high-value mechanisms that were effectively untested in both implementations. This PR adds matched TypeScript + Go test pairs for them, plus one small parity bugfix the new tests revealed.

Tests added

Area	TypeScript	Go
Build cache (cacheHit/cacheSig)	cache.test.ts — hit reuses model, change re-resolves, error not masked by cache	build_test.go — TestCacheHitSkipsResolve, TestInvalidateCacheForcesResolve
CLI	cli.test.ts — missing-file, no-args, bad-model exit non-zero	(already covered in main_test.go)
Watch	watch.test.ts — add/remove mode registration, descDeps edge cases, start() with config disabled	model_test.go — TestWatchDetectsNewFile, TestStartWithoutConfig
Parity	parity.test.ts — second shared fixture (arrays of objects)	parity_test.go — matching …ArrayOfObjects
Config order	—	config_test.go — fallback to sorted keys when no explicit order.action

The build cache (the mtime-based incremental-rebuild skip at the heart of watch mode) was 0% covered in Go and untested in TS — the highest-risk gap and a prime parity-drift spot, since each implementation computes the signature independently.

Bug fixed

Writing the CLI exit-code tests revealed that the TS CLI exited 0 even on a broken model, while the Go CLI (and docs/reference.md, which documents exit 1 "on … a build error") expect non-zero. Fixed ts/bin/voxgig-model to propagate a failed build as a non-zero exit code. This is the only behavior change here; it aligns the implementation with both the docs and the Go CLI, and is locked down by the new bad-model-exits-nonzero test.

Coverage

• Go library: 91.9% → 95.0% (cacheHit 0% → 100%, configOrder 53.8% → 92.3%)
• TS: lines 94.6% → 95.7%, branches 83.2% → 86.3%, funcs 92.8% → 94.2%
• TS suite 26 → 36 tests

Verification

• make test green in both languages
• gofmt -l go clean; go vet ./... clean
• ts/dist-test/ regenerated and staged; no ts/dist/ source drift; no _gen/ artifacts staged

🤖 Generated with Claude Code

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