test(buildgen): add incremental output equivalence

[cssbruno]

Summary

• adds a reusable buildgen equivalence harness that compares incremental SPA output to a clean build of the same final source state
• compares served output plus sibling compiler report output, exact bytes, file modes, manifests, and a narrow canonical projection of build-report artifact events
• removes stale generated asset files that are no longer present in the current asset manifest during incremental publication
• documents the [Testing] Add incremental-versus-clean build equivalence testing #686 invariant, current normalization rule, first deterministic scenario, and remaining follow-up coverage

Validation

• go test ./internal/buildgen -run 'TestIncrementalBuildMatchesCleanBuildEquivalence|TestBuildIncremental' -count=1
• go test ./internal/buildgen -count=1
• go build ./cmd/gowdk
• scripts/check-docs-style.sh (passes; existing long-paragraph warnings reported)
• scripts/check-docs-links.sh
• go test ./... (fails in internal/gowdkcmd: TestReleaseTrustWorkflowCoverage expects missing .github/workflows/release-smoke.yml; .github/workflows/cache-maintenance.yml is also absent in this checkout)

Issue

Refs #686

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve assets still referenced by unchanged pages

In BuildIncrementalFromValidatedProgram, only pages listed in changedPageSources are re-rendered, but this cleanup removes every previous manifest asset that is not in the current asset set. When a global CSS processor asset such as Tailwind's app stylesheet changes during a single-page incremental rebuild, unchanged pages keep their old hashed stylesheet link while this deletes that old file, turning those pages' CSS URL into a 404; either force all pages that include changed global assets to render or delay deleting stale assets until no unchanged HTML can reference them.

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