Phase 536: lasso screenshot route and Cookbook proof boundary

[ExplodingUFO]

Context

Phase 535 refreshed the post-lasso visual feedback parity queue and selected this as the next bounded whiteboard follow-up.

Phase 534 already added transient lasso visual gesture feedback, but the Cookbook/screenshot proof boundary still records lasso screenshot coverage as a gap. The next slice should make the lasso route demonstrable and screenshot-trackable without expanding into toolbar UX or broader whiteboard implementation.

Scope

Add the first tracker-backed lasso screenshot proof boundary:

• define the Cookbook/demo fixture route or state needed to expose the existing public lasso selection mode plus transient lasso feedback in a screenshotable scenario;
• extend the screenshot manifest or focused visual gate tests for that lasso proof route;
• update bilingual React Flow parity docs and focused docs tests to record Phase 536 as owning the lasso screenshot route boundary;
• keep the proof tied to existing lasso behavior from Phases 530, 533, and 534.

Out of scope

Toolbar UX, new public API, eraser behavior, rectangle/freehand drawing primitives, persistence, renderer-layer rewrite, strict pixel-baseline enforcement, retained API removal, and any full React Flow whiteboard parity claim.

Dependencies

• Closed: Phase 535 / GitHub Phase 535: refresh post-lasso visual feedback parity queue #193 / avalonia-node-map-8l6.
• Sequential with future Phase 537 if both need the same lasso Cookbook route copy or visual proof fixture.

Acceptance Criteria

• A Cookbook/demo lasso proof route or manifest state exists and is covered by focused tests.
• The screenshot/visual gate evidence explicitly targets lasso selection visual feedback rather than generic shell coverage.
• EN/ZH parity docs record Phase 536 with this GitHub issue and its Bead ID.
• The Next Issue Wave table removes Phase 536 as a TBD candidate while preserving Phase 537-540 follow-ups.
• Tests prove Phase 536 does not claim toolbar UX, eraser, drawing primitives, persistence, renderer rewrite, strict pixel baselines, retained API removal, or full whiteboard parity.