feat(wait): add bounded Chromium network-idle predicate

[SarthakWade]

Summary

• add wait --network-idle as an optional predicate that composes with URL, text, and settled waits
• track bounded Chromium CDP requests with a 500 ms quiet window and explicit persistent-connection exclusions
• report the feature through capabilities and fail immediately with UNSUPPORTED_CAPABILITY on WebKit
• update the CLI, protocol schema, generated SDKs, agent skill, architecture decision, and command documentation

Safety and compatibility

• preserve the existing bare wait settled-page behavior and wire request
• keep request URLs, identifiers, headers, bodies, and timings out of wait responses
• cap tracking at 4,096 request IDs and 512 bytes per ID, failing closed on overflow
• keep all waits bounded by the existing timeout contract

Validation

• pnpm test
• pnpm test:runtime
• pnpm test:npm
• Python SDK: 42 pytest tests, mypy, ruff check, and ruff format check
• pnpm --filter @headless/app build
• pnpm --filter @headless/web lint
• pnpm --filter @headless/web build
• pnpm test:e2e:mac
• Linux protocol suite passed in Docker; local browser E2E image setup was blocked by repeated deb.debian.org connection timeouts and is left to the required CI gate

Closes #204

[yashranaway]

wait --network-idle is additive and engine-honest.

Bare wait still means settled. Chromium tracks CDP request IDs only, with a 500 ms quiet window, 4,096 ID cap, 512-byte ID cap, and overflow that never reports idle. WebSocket and EventSource are excluded. Wait responses do not grow request URLs, headers, or bodies.

WebKit returns UNSUPPORTED_CAPABILITY immediately instead of using page-world diagnostics. The capability bit matches that. macOS E2E asserts the typed failure; Linux E2E covers timeout and a successful quiet wait.

CI is green.