feat(select): add semantic native dropdown selection

[SarthakWade]

Summary

• add headless select for native single-selection HTML controls using semantic or inspected-ref targets
• support one exact whitespace-normalized label or exact value matcher
• expose the command consistently through both browser engines, the canonical protocol schema, generated SDKs, capabilities, docs, and the browser-use skill

Safety and behavior

• reject custom ARIA comboboxes, multi-selects, disabled controls, disabled options and groups, stale refs, missing options, and ambiguous matches before mutation
• dispatch input then change only when the selected option changes
• keep repeated selection idempotent
• mark option matchers sensitive, omit them from responses, and exclude select commands from flow recording
• retain the no-arbitrary-JavaScript boundary by using one fixed isolated-world operation

Verification

• pnpm test (71/71 protocol tests, secure prompt, MCP)
• pnpm test:runtime
• pnpm test:npm (41/41)
• Python SDK: pytest (42), mypy, Ruff check, Ruff format
• pnpm --filter @headless/web lint
• pnpm --filter @headless/web build
• pnpm --filter @headless/app build
• pnpm test:e2e:mac completed the full P2 flow once; two later repetitions passed the expanded select cases and then hit the existing durable-authentication local-storage persistence flake
• shell syntax checks and git diff --check

Local Linux browser E2E reached and passed the Linux protocol suite, but Docker could not reach the Debian package mirror while constructing the browser test layer. CI remains the authoritative Linux browser run.

Merge order

Merge #205 before this PR. Its architecture decision is #34 and this PR intentionally follows it as #35.

Closes #206

[yashranaway]

select stays inside the existing isolated-world boundary. It only mutates a native single-selection HTMLSelectElement, fails closed on multi-select, disabled controls/options, missing and ambiguous matchers, and does not advertise custom comboboxes.

Matchers are sensitive schema fields, omitted from the response, and excluded from flow recording. Both engines declare selectDispatch: synthetic-dom instead of pretending this is trusted CDP input.

Merge #205 first so ADR numbering stays 34 then 35. CI is green.

[yashranaway]

Re-approve after merging main. ADR 34 and 35 both present; schema includes wait --network-idle and select.