G6: add bounded region-scoped screenshot series

[SarthakWade]

Parent: #54, roadmap backlog G6.

Goal

Let an agent turn an outline region reference into a bounded screenshot series without selectors or unbounded full-page capture. The original backlog names Linux, but the existing portable series pipeline makes cross-engine behavior the safer contract.

headless --session qa inspect --context outline
headless --session qa screenshot --by-region @r4 --output checkout

Contract

• Add screenshot --by-region @rN as a third series mode alongside --every-viewport and --by-section.
• Accept only a region reference issued by the current page runtime. Do not accept selectors, element refs, roles, or names in this mode.
• Capture only the region bounds. Split a tall region into deterministic vertical image slices and include the final extent. Do not include pixels outside the region.
• Preserve the current PNG/JPEG formats, private artifact store, exclusive creation, numbered output prefixes, 80-image cap, truncated, and totalPoints reporting. PDF and clipboard remain unsupported for series.
• Revalidate the region before each slice. Unknown, expired, detached, hidden, non-intersecting, or invalid geometry fails closed with REGION_NOT_FOUND or the existing bounded-capture error.
• Reserve the complete artifact set before capture and discard every reservation on failure. Never return a partial successful series.
• Restore the original scroll position after success or failure.
• Apply the shared screenshot dimension and pixel bounds to every slice.
• Mark series metadata containing page-derived labels as untrusted. Do not add page text, selectors, HTML, coordinates from the runtime map, or region contents to logs or flows.
• Advertise region series explicitly in capabilities and generated SDK metadata. Keep protocol version 0.5 if the change remains additive.

Required implementation surfaces

• Architecture decision before behavior changes.
• CLI parser/help, protocol validation, Rust protocol mirror, canonical schema and fixtures.
• Isolated runtime region planning and per-slice bounds.
• Shared series orchestration and both WebKit and Chromium adapters.
• TypeScript and Python generated SDK contracts and validators.
• README, command reference, phase contract, skill docs, and G6 backlog status.

Acceptance criteria

• Parser and protocol tests cover valid refs plus unknown, malformed, conflicting, PDF, clipboard, single-output, full-page, element-target, and mixed-series cases.
• Runtime tests cover short and tall regions, final-slice coverage, the 80-image cap, stale/detached/hidden refs, invalid geometry, and layout changes during capture.
• Shared tests prove atomic reservation cleanup and scroll restoration on success and failure.
• Linux and macOS E2E each prove a real region series contains only region pixels and produces deterministic bounded artifacts.
• Existing single, full-page, viewport, and section screenshots remain unchanged.
• Protocol, runtime, SDK, Rust, build, and both engine E2E gates pass.