feat(#27): add bounded Windows Terminal active-tab guard

[rblake2320]

Scope

Adds the bounded Windows Terminal active-tab guard tracked by #27.

• Retains the live UIA TabItem and binds its opaque RuntimeId, selected state, focused TermControl RuntimeId, guarded top-level target, and peer birth_id into the immutable operation and signed peer ACK.
• Uses SelectionItem.Select, CompareElements, and before/after checkpoints around every native body batch and Enter.
• Treats pre-call drift as refusal and post-call drift as ambiguous.
• Packages the implementation, contract tests, live drill, redacted result, and bounded claim documentation.

Evidence

• python -m pytest -q: 830 passed, 10 skipped.
• Focused active-tab/guard tests: 168 passed.
• Ruff: clean.
• python -m build: sdist and wheel built; wheel contains the module, drill, result, and documentation.
• Real controlled duplicate-title Windows Terminal drill: PASS for retained identity after reorder, background-window denial, wrong-tab denial, split-pane TermControl mismatch, post-call ambiguity, close denial, and reopen denial.

Boundary

This does not claim exclusive per-tab hardware or message routing. UIA selection and later PostMessage/SendInput calls are not atomic. RuntimeIds are desktop-session opaque and reusable; title and tab index are never identity. Exact preventive routing remains a separate birth-ID-bound named-pipe/control-plane transport.

Issue #27 remains open pending independent review and merge evidence.

Review amendment

Fails closed unless exactly one TextPattern candidate contains the focused UIA element; document-length fallback removed. Native-batch entry remains ambiguous through result validation and the post-call checkpoint; explicit returned zero acceptance remains refused. Caller-asserted birth_id is digest-bound, not live-registry-attested by this module.

Strict zero refusal amendment: only the exact typed CASCADIA postmessage result schema (ok=false, requested=1, accepted=0, target-bound, no missing/extra fields) clears native-entry ambiguity. Seven malformed/inconsistent zero-result vectors remain ambiguous.

Production-integration amendment: _production_send_body now emits the canonical typed delivery-result schema for class/override denial, deadline, PostMessage rejection, and success. A real monkeypatched _send_char_postmessage=False result passes the exact-zero classifier and the guarded-submit integration refuses; malformed zeros remain ambiguous.