feat(snapshot): honor --depth=N with landmark nesting

[drakulavich]

Summary

• The snapshot script now emits a path of landmark ancestors (main, navigation, header, footer, section, article, aside, form, dialog, list, region, …) per ref. toYaml renders landmarks as parent nodes and reuses shared prefixes between sibling refs, producing real aria-tree nesting that matches playwright-cli output.
• --depth=N is honored: --depth=1 reproduces the flat v0.2 output, --depth=2 keeps only the outermost landmark, default (omitted) is unbounded. --depth=0 is rejected with a usage error.
• Crosses off the next open roadmap item ("Snapshot nesting honoring `--depth=N`") and unblocks the AGENTS.md gotcha that said "flat in v0.2 — fix the parser, not callers, when implementing nesting."
• Ref.path is optional, so older state.json files from v0.2 keep working and render flat.

Example

A page with a primary nav, a signup form inside <main>, and a footer now renders:

- generic:
  - navigation \"Primary\":
    - link \"Home\": [ref=e1] /
    - link \"About\": [ref=e2] /about
  - main \"\":
    - form \"Sign up\":
      - textbox \"Email\": [ref=e3]
      - button \"Submit\": [ref=e4]
  - contentinfo \"\":
    - link \"Contact\": [ref=e5] /c

bowser snapshot --depth=2 clips that to:

- generic:
  - navigation \"Primary\":
    - link \"Home\": [ref=e1] /
    - link \"About\": [ref=e2] /about
  - main \"\":
    - textbox \"Email\": [ref=e3]
    - button \"Submit\": [ref=e4]
  - contentinfo \"\":
    - link \"Contact\": [ref=e5] /c

Test plan

• `bun test` — 77 pass / 0 fail / 10 skip
• Unit golden tests for nested rendering and depth clipping (`tests/snapshot.test.ts`).
• Command-layer test that `cmdSnapshot` honors `--depth` and rejects `--depth=0` (`tests/commands.test.ts`).
• `BOWSER_E2E=1 bun test tests/e2e.test.ts tests/e2e-todo.test.ts` against a real headless Chromium — both pass; the existing substring assertions (`'"Add": [ref='`, etc.) still match because leaf lines still end with the same `role "name": [ref=eN]` pattern, just under deeper indentation when relevant.
• `bun build src/cli.ts --compile` produces a working binary; `--help` still lists `snapshot [--filename=f] [--depth=N]`.
• (Not run here) live-internet `BOWSER_E2E_NET=1` against github.com.

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