test: verify exact Loom revision links (Refs #14)

[Devesh36]

Change and scope

Refs #14. Adds browser fixture coverage for Loom's issue contribution entry: an exact deep link fetches and displays its requested Pattern revision, and a retired/unavailable revision is surfaced as an alert without redirecting elsewhere. The existing implementation already provides these behaviors; this contribution makes them regression-tested.

Verification

• npm --prefix clients/web run test -- src/lib/components/LoomView.test.ts — 7 passed.
• make frontend-check — Svelte diagnostics clean; 171 tests passed.
• git diff --check — clean.

Documentation and delivery claims

None. This test-only change adds evidence for existing behavior and does not alter delivered behavior or documentation claims.

Author review

• I reviewed the full diff, including any AI-assisted changes, and can explain the submitted code and its verification.