fix(frontend): ignore stale blueprint loads after route changes

[ai-yang]

Fixes #146

Summary

• Ignore getBlueprint() callbacks after their route ID or RPC stub has been superseded.
• Tie rendered blueprint data to the (id, rpcStub) that loaded it, so the old blueprint cannot flash during the first render of a new route.
• Clear loaded/error state at the start of each load.

Without these guards, an older request could settle after the current request and overwrite the page. The URL and action handlers would then target the new blueprint ID while the UI displayed metadata from the old blueprint.

Regression coverage

The new component tests use controlled promises to verify that:

• an A response resolving after B cannot replace B;
• the first render after changing A to B shows loading rather than A;
• a not-found B response does not leave A visible; and
• a rejected B request shows B's error without leaving A visible.

Test plan

• pnpm --filter @gadgets/workshop-frontend test — 28 test files / 140 tests passed
• pnpm --filter @gadgets/workshop-frontend types:check
• pnpm exec oxlint packages/workshop-frontend/src/BlueprintLandingPage.tsx packages/workshop-frontend/src/BlueprintLandingPage.test.tsx
• git diff --check

[github-actions]

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[ai-yang]

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