feat: appetizer latency metric + new LA click tracking

[yodem]

Implements two Library Assistant stories, based on the appetizer/progress-trail work in PR #136 (waiting-source).

sc-44755 — time_to_appetizer Braintrust metric

Adds a numeric, aggregatable metrics.time_to_appetizer on the trace measuring user-perceived latency from request receipt to the moment the appetizer is served (seconds, one decimal). Only emitted when an appetizer is actually served (suppressed/error/no-result/stream-closed all skip it). The existing metadata.appetizer debug blob is kept.

Note on semantics: the metric stops at server-side enqueue (progress_queue.put), so it reads slightly lower than the PR #136 "appetizer visible 5.1s" (client paint). Happy to move it to SSE-flush if preferred — flagged on the story for discussion.

sc-44684 — new assistant_click feature_names

Reports four new click feature_names. Implemented via a dedicated data-feature-name attribute read by the host click tracker before the generic link/aria-label logic — chosen over the ticket's literal "aria-labels" because:

• the context chip already has a real, dynamic aria-label (overwriting it would degrade screen-reader output);
• the appetizer/thinking links are <a> elements that otherwise short-circuit to the generic "Response link" label.

Reporting is identical either way — GA4 keys on the feature_name value, not the source attribute.

Element	feature_name
Context chip (reading ref)	initial_location_link
Appetizer topic links	related_topics_link
Thinking-steps toggle	thinking_steps_toggle
Thinking-step ref links	thinking_steps_text_link

Also drops stopPropagation on the appetizer link (kept preventDefault) so the click reaches the host tracker. Only document-level click listener is the menu outside-click-close, which is unaffected.

Notes

• The context-chip commit (initial_location_link's target element) is carried in as a dependency — it isn't in PR feat: Progress Trail + Topic Appetizer for latency reduction #136. The other three labels + the metric are native to PR feat: Progress Trail + Topic Appetizer for latency reduction #136's feature set.
• Per Sefaria convention, no Shortcut story IDs appear in source.

Test plan

• Backend: deterministic integration test asserts metrics.time_to_appetizer is emitted (runs appetizer inline to avoid thread races). Full suite 45 passed.
• Frontend build passes.
• Cauldron validation outstanding — story sc-44684 requires validating the four feature_names fire in cauldron before deploy.

🤖 Generated with Claude Code

[gitvelocity-reviewer]

📊 Code Quality Score: 44/100

55 × 0.8 = 44

Category	Score	Factors
🔭 Scope	13/20	7 files across frontend (Svelte ×3), backend (Python ×2), i18n, tests; new context chip feature, metric promotion, UI restyling; cross-cutting analytics/i18n/observability
🏗️ Architecture	10/20	Global history.pushState monkey-patch (notable risk); data-feature-name analytics pattern is clean; metric promotion from metadata to numeric; no new services or major structural changes
⚙️ Implementation	12/20	URL parsing with multiple fallback paths; SPA navigation detection via pushState/replaceState patching; derived reactive state; CSS tooltip with pseudo-element transitions; reasonably complex but not algorithmically deep
⚠️ Risk	8/20	Global history.pushState mutation is permanent and affects all navigation; stopPropagation removal changes event flow; no database/auth changes; frontend changes are easily reversible
✅ Quality	10/15	Good integration test for time_to_appetizer metric with deterministic _SyncExecutor pattern; no frontend tests for context chip or history patching; some removed comments reduce documentation; i18n strings added
🔒 Perf / Security	2/5	Analytics is non-blocking; noopener/noreferrer on external links; no security concerns introduced; no benchmarks or threat model

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[yodem]

Superseded by two focused per-story PRs off waiting-source: #151 (appetizer latency metric) and #152 (new LA click types, 3/4 — initial_location_link deferred until the context chip lands). This combined PR also carried unrelated context-chip code; the split removes it.