Phase 538: eraser behavior/API feasibility gate

[ExplodingUFO]

Context

Phase 537 is implemented in PR #198 but is blocked by required review. The parity roadmap still identifies the next independent whiteboard/lasso candidate as Phase 538: eraser behavior/API feasibility gate.

React Flow's whiteboard track treats Eraser as a distinct advanced behavior. AsterGraph currently has graph selection/delete commands and lasso/marquee selection evidence, but it must not claim eraser parity until the API and behavior boundary is explicitly analyzed and guarded.

Scope

Create a bounded feasibility gate for eraser behavior/API that answers what AsterGraph can safely expose next without inventing a broad whiteboard system:

• inspect existing selection/delete commands and Avalonia hit-testing seams;
• decide whether eraser should remain a documented gap, become a command/API proposal, or get a minimal tracker-backed route;
• add focused docs/tests that record the exact boundary and prevent accidental eraser parity claims;
• update EN/ZH parity docs and public API/Cookbook guidance only as needed;
• avoid overlapping Phase 537's hosted toolbar implementation and avoid changing NodeCanvas.SelectionMode unless the gate proves it is necessary.

Out of Scope

No drawing primitives, no persistence, no renderer rewrite, no strict pixel baselines, no retained API removal, no full whiteboard parity, and no broad UI redesign. Do not implement a full eraser cursor/trail unless this gate first proves the minimal API/behavior contract and verification route.

Acceptance Criteria

• The repository records Phase 538 with this GitHub issue and bead ID in EN/ZH parity docs.
• Focused tests prove the eraser feasibility boundary and fail if docs claim full React Flow whiteboard/eraser parity prematurely.
• Existing selection.delete behavior is either explicitly reused as graph-selection deletion evidence or separated from eraser tool behavior with clear wording.
• Next Issue Wave removes Phase 538 as TBD while preserving Phase 539-540 as future tracker-backed work.
• Verification includes focused docs tests plus any focused editor/Avalonia tests required by the chosen boundary.

Dependencies / Parallelism

Can run while PR #198 waits for review if the implementation avoids Phase 537's hosted toolbar files and avoids shared pointer-mode state edits. It should not merge over PR #198 without checking current master and the PR state.