fix(ci): restore documented CI job graph in test.yml

[pythoninthegrass]

Summary

• build and playwright-tests had drifted to gating on rust/vitest-tests (and playwright-tests for build) in addition to deno-lint, producing a serial rust -> playwright -> build chain that docs/builds.md never documented and that costs up to ~40 min of wall-clock per run.
• Narrows build's needs: to [deno-lint] and playwright-tests' needs: to [changes, deno-lint] — matching what docs/builds.md's "Test Workflow Dependency Graph" section already claimed.
• Updates that same doc section, which was stale in two ways: it said "five jobs" (now eight, after zig/shadow-diff/changes were added) and listed playwright-tests as fully independent when it was already gated on deno-lint.

Why this is safe

Neither build nor playwright-tests consumes artifacts from rust or vitest-tests: build runs its own per-platform cargo check, and playwright-tests runs against a Vite preview build with mocked IPC.

Before/after (TASK-351 AC#3, AC#4)

Real PR-run data pulled via gh run view --json jobs, not synthetic:

Before (run 34811970745, TASK-355.6's PR, pre-fix graph):

Job	Start	End	Result
Deno Lint and Format Check	06:05:14	06:05:23	success
Rust Lint, Format, and Test	06:05:06	06:06:53	failure (pre-existing mt-zig-core staging bug, unrelated to this change)
Playwright E2E Tests	06:06:54	06:06:53	skipped — cascaded from rust/vitest-tests failure
Build (matrix)	06:06:54	06:06:54	skipped — cascaded from rust/vitest-tests/playwright-tests

build and playwright-tests never ran at all — not delayed, dropped. That's the actual cost of the drifted graph: a single unrelated rust job failure silences build/E2E signal for the whole PR.

After: see this PR's own run once CI completes — build and playwright-tests should now start immediately after deno-lint succeeds regardless of rust's outcome.

Test plan

• actionlint .github/workflows/test.yml — clean
• This PR's own Actions run confirms build starts within ~3 min of deno-lint completing (AC#3)
• docs/builds.md dependency graph re-verified against test.yml post-merge (AC#5)