[frontend-contrib S3] P3 proof: dependency-island build matrix + plugin-vite pin policy

[rickylabs]

Part of #922

Scope

Prove fresh({ islandSpecifiers }) (plugin-vite 1.0.8) and Builder.registerIsland for plugin-package islands: clean cache x {local-source, jsr} x {dev, prod build+SSR+hydrate}; single Preact/Signals identity; dependency CSS; edit-loop behavior (HMR vs signaled reload); ^1.0.8 drift policy.

Design source: RFC #890 (rfc.md §5) + .llm/runs/plan-frontend-contrib--seed/design/canonical/ (rev 3).

Acceptance

• gate: full matrix executed with recorded results per cell
• gate: plugin-vite pin/compat decision recorded