rivet serve - artifact view

[avrabe]

in arifact view in rivet serve i see the file it is in but clicking on it does not open the file with the location even thoug hwe have sources.
currently only linked in ther vsix extension

[avrabe]

Triage from the scheduled routine.

Clear ask — rivet serve's artifact view shows the source file an artifact lives in, but the path is not a click target that opens the file at the right location. The vsix extension already has the equivalent affordance; serve is the gap. Routine needs an explicit AC block before opening a PR; suggested shape:

## Acceptance Criteria
- [ ] In `rivet serve`'s per-artifact view, the rendered "Source" path becomes an anchor whose href opens the source file at the artifact's start line. Behaviour matches the existing vsix extension's affordance.
- [ ] Link target (pick one — document in the PR): (a) `vscode://file/<abs-path>:<line>` URI handler (works on systems where VS Code registered the handler — the same posture the vsix extension assumes); (b) a serve-side endpoint `/source/<rel-path>?line=<n>` that returns the file rendered with the artifact's range highlighted, opened in a new tab — works without any external editor.
- [ ] Both `start-line` and (if known) `end-line` come from the existing artifact `source` metadata (rowan CST spans — same field the per-artifact view uses to show the path today). No new parsing pass.
- [ ] Static export (`rivet export --format html`): the same path is rendered as a non-clickable label in static export (no `vscode://` link from a static report — those report URLs leak local paths and don't resolve off the author's machine). Acceptance asserts the link is omitted in the export's golden file.
- [ ] Regression: opening the rivet repo's own dashboard, clicking the Source line of any artifact deep-links to its file:line (Playwright fixture).
- [ ] Commit trailer: `Implements: REQ-NNN` (new REQ for "serve-side source deep-linking") + `Refs: FEAT-001, #623`.
- [ ] **Explicitly out of scope (file separately):** a generic "open in $EDITOR" preference (Helix / JetBrains / Emacs — separate UX call); editing the source from the page (#546 territory).

Related: #546 (editor / UI for human-friendly editing), #547 (graphical req→test trace), #621 (backend-down UX), #622 (overview-level diagnostics).

Not implemented this run.

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