feat: add bounded region screenshot series

[SarthakWade]

Summary

• add screenshot --by-region @rN as a bounded, cross-engine screenshot series
• bind captures to the current document and exact region geometry, with per-slice viewport and clipping revalidation
• reserve artifacts atomically, restore scroll position, report explicit truncation, and omit page labels and coordinates from region metadata
• update capabilities, generated TypeScript and Python SDK contracts, the Rust validator, command docs, roadmap, and agent skill

Security and failure behavior

• accepts only current-page opaque region references, never selectors or arbitrary JavaScript
• rejects stale, hidden, clipped, moving, oversized, malformed, or non-ASCII references
• caps region width at 4096 CSS pixels, keeps shared image bounds, and limits output to 80 artifacts
• removes every reserved artifact on capture, validation, write, or scroll-restoration failure
• revalidates the viewport after capture to reject scroll races

Verification

• pnpm test: 73/73 protocol tests plus MCP and secure prompt tests passed
• pnpm test:runtime: passed
• pnpm test:npm: 41/41 passed
• Python: Ruff, format, strict mypy, and 42/42 pytest passed
• Rust: cargo test --locked and strict Clippy passed in Docker
• pnpm build: macOS app and website passed
• pnpm test:e2e:linux: passed with real Chromium and all three region artifacts validated
• targeted production WKWebView E2E: passed with all three region artifacts validated
• full local macOS E2E reaches an existing System Events accessibility failure at start-page-address-hud before screenshot coverage; the PR carries macos-e2e for the clean runner

Closes #211

[yashranaway]

Thermo-nuclear + deslop pass.

Clean bounded surface screenshot --by-region @rN, opaque region refs only. Reuses artifact reservation + viewport revalidation + scroll restore, no fork. Protocol-first, SDKs + schema + fixtures in sync. No file >1k due to PR, validation centralized.

Nits (non-blocking):

• HostCore.swift mixes reservation/restore + capture — extract RegionCaptureCoordinator if more region ops land
• AgentRuntime viewport math duplicated — watch drift

Verification strong (protocol/MCP/python/rust + e2e). Ready to merge after other reviewer.