feat(plugin): add multi-angle grid generation

[superche]

Summary

• Add the multi-angle v0.1.0 provider-neutral Web Plugin and register it at Registry sequence 27.
• Discover compatible installed image tools through generation.tools.list and submit exactly one generation.canvas.execute request for the direct incoming reference image.
• Build one prompt that asks for a single equal-cell multi-angle grid image, with one consistent subject and one requested viewpoint per cell.
• Persist portable run state and authoritative generated Canvas node ids, including interruption, failure, retry, and stale-result handling.

Why

The product Plugin now belongs in microvoid/convax-plugins; Convax itself keeps only the generic Plugin host and generation capabilities. The Plugin does not contain provider/model credentials or branch on a concrete generation vendor.

Paired host reconciliation fix: https://github.com/microvoid/convax/pull/40

Validation

• bun install --frozen-lockfile --ignore-scripts
• bun test tooling/multi-angle.test.js tooling/registry.test.js — 13 passed, 159 assertions
• bun test — 231 passed, 1,278 assertions
• Package builds, source validation, workspace typechecks, and workspace tests passed
• bun run pack — produced convax-plugin-multi-angle-0.1.0.zip (66,597 bytes)
• bun run build:index — Registry sequence 27 with 18 packages

Local validation note

bun run check reaches the unchanged FFmpeg companion smoke and fails twice with Compiled companion did not return the expected source.mp4 generation result. This branch does not modify packages/tools/ffmpeg-mcp; all remaining root tests, Plugin packaging, and Registry index generation pass independently.

The GitHub macOS 14 full check completed successfully for this PR, so the local FFmpeg smoke result is not a CI blocker: https://github.com/microvoid/convax-plugins/actions/runs/29980792313