fix(cua-driver): bound large macOS text synthesis

[f-trycua]

Summary

• keep large atomic macOS AX text insertion uncapped
• estimate the actual Unicode or physical synthesis route before posting character events
• refuse over-budget synthesis before mutation with structured chunk or verify-state guidance
• report observable partial AX delivery without appending the full payload again
• refresh the generated MCP reference with the bounded-delivery contract

Why

The macOS synthesized fallback has a per-character floor plus a bounded delivery drain. Large payloads can therefore outlive the daemon's 120-second request deadline, leaving callers with a transport timeout while the target may still be partially mutating.

This change gives synthesized routes a documented 100-second scheduled-work budget, reserving the remaining transport time for event construction, routing, focus assistance, queueing, response serialization, and transport delivery. Atomic AX insertion remains the preferred one-call path and is deliberately exempt because its cost does not scale per character.

Fixes #2861

Regression coverage

• exact transport-budget boundary and one-character-over refusal
• large atomic AX payload remains uncapped
• terminal and foreground routes refuse before invalid target/event work
• structured safe-chunk versus indeterminate-AX diagnostics
• partial AX progress returns the delivered prefix instead of replaying the complete payload
• web AX ambiguity requires state verification rather than blind retry

Verification

Exact source: 358bd55dd1dde03df1aab246d8b7d72a169e9f8a

• cargo test -p platform-macos — 312 passed, 0 failed; 1 doctest ignored
• focused type_text tests — 19 passed, 0 failed
• cargo fmt --all -- --check
• git diff --check origin/main...HEAD
• npx tsx scripts/docs-generators/runner.ts --library cua-driver --check
• generated contract/SDK parity passed on macOS, Linux, and Windows in CI

Native macOS certification

Certified from the exact source SHA on a disposable macOS 26.5.2 arm64 Lume guest using a signed Cua Driver 0.17.0 candidate.

• installed binary SHA-256: 5a4b0aea4a3cf971677631f7ac17767064afa65e14ded178facd746e289423d7
• designated requirement leaf: d78483e2cab099da4b14bed941d479b902fee005
• get_config.source_sha matched the exact PR head
• the driver reported Accessibility and Screen Recording granted under its own responsible app identity
• a 3,000-character foreground Electron request at 30ms/character returned type_text_synthesis_budget_exceeded in 1.1 seconds with delivered_chars: 0, synthesized_chars: 0, and a safe chunk limit of 2,578 characters
• a fresh accessibility snapshot proved the real Electron field remained exactly type here after refusal
• a subsequent 17-character foreground request reported all 17 delivered and the real field contained the marker, proving the daemon and normal route remained responsive
• the reporter's smaller-payload concern was also exercised: 300 synthesized characters at 0ms completed in 7.5 seconds, the renderer remained healthy with the full 218-element tree, and all 300 characters were observable
• the harness field and original signed guest installation were restored after certification; the exact candidate was retained in the guest evidence cache