C7: add semantic native select command

[SarthakWade]

Parent: #35

Problem

Headless can inspect native <select> controls but does not advertise or execute a selection action. Agents must fall back to fragile key presses, and the current inspection contract deliberately omits a nonexistent select hint.

Contract

Add a portable select command for native, single-selection HTML <select> elements:

headless --session qa select @e4 --label "Canada"
headless --session qa select --role combobox --name Country --value CA

• Reuse the existing target grammar: exactly one element ref or semantic role/name target.
• Require exactly one option matcher: --label or --value.
• Match option labels after the runtime's existing whitespace normalization; match option values exactly.
• Require exactly one matching option. Missing or ambiguous matches fail without changing selection.
• Reject non-native controls, multiple selects, disabled selects, disabled options, and options inside disabled optgroups.
• Dispatch input followed by change only after validation and mutation succeed.
• Return bounded target metadata and selected option index, but do not echo page option labels or values.
• Mark page-derived output untrusted.
• Do not add select to recorded flows in this change.

Custom ARIA comboboxes, listboxes, multi-select, hover, drag, and arbitrary evaluation remain out of scope.

Engine behavior

Both engines use the fixed isolated-world runtime operation because there is no portable native menu API with equivalent behavior. Add a separate selectDispatch: synthetic-dom capability so Chromium's existing inputDispatch: trusted-cdp claim remains accurate for click, fill, and press.

Required changes

• Record the contract in docs/roadmap/architecture-decisions.md before changing behavior.
• Add command enum, validator, CLI parser/help, protocol schema/result, HostCore dispatch, and both engine adapters.
• Add the isolated runtime operation and advertise select only for eligible native single-select controls.
• Regenerate TypeScript and Python SDK contracts.
• Update command docs, phase docs, skill references, and the C7 backlog entry without closing C7 — Design review for deferred command ideas (hover, drag, select, isolation) #35 while hover and drag remain open.

Tests

• Parser and protocol tests for target conflicts, matcher exclusivity, bounds, and generated-schema drift.
• Runtime tests for ref and semantic targeting, label/value selection, event order, stale refs, and every no-mutation rejection path.
• macOS and Linux E2E tests using a real native select, including one happy path and representative disabled, ambiguous, multi-select, and custom-combobox failures.
• Capability tests proving both engines declare synthetic select dispatch.

Definition of done

All protocol, runtime, SDK, web, macOS, and Linux E2E gates pass; docs and generated contracts are current; no unsupported control is advertised as selectable.