feat(panel): keep huge sessions navigable — transcript windowing + delegated collapse

[upsetbit]

Problem

Opening a very large session in the panel (e.g. 1.9B tokens, hundreds of thousands of turns) froze the browser tab. Two compounding causes:

1. The side panel rendered the entire transcript at once — Sessions.Get had no turn windowing (SELECT … FROM turns with no LIMIT), so every turn landed in the DOM.
2. Every bubble was a live Alpine component with its own x-data and a window-level listener; tool/thinking groups too. Thousands of turns meant thousands of reactive components initialized on load, and "Collapse all" / in-transcript search dispatched synchronous O(N) event storms.

What changed

API (Sessions.Get) — new optional turn_limit / turn_offset / turn_tail request fields window the chronological turn stream server-side; the response now carries total_turns and message_turns. turn_limit = 0 keeps the old return-everything behavior, so existing callers are unaffected.

Transcript paging (panel) — the side panel requests only the last 200 turns (turnPageSize) and opens on the tail of the conversation. A "↑ load earlier turns (N more)" anchor at the top of the transcript HTMX-swaps in the preceding 200-turn page (GET /sessions/<id>/turns?before=N), stacking older pages above the ones on screen until turn 0. The "turns" KPI uses the server-side message_turns count, so it stays whole-session-accurate regardless of the window.

Delegated collapse (panel) — per-bubble collapse, tool/thinking group disclosure and "Collapse all" are now plain classes + [hidden] flipped by delegated listeners in assets/transcript.js: a constant number of listeners per document instead of one Alpine component + one window listener per message. The in-transcript search keeps working through the same messages-toggle-all / transcript-expand-groups events it already dispatched. Bubble visuals are unchanged.

Validation

Exercised end-to-end against a disposable Postgres/MinIO stack with a synthetic 1,260-turn session (plus a small control session):

• Initial render: ~1.6k DOM nodes and 1 Alpine component in the panel (the search box), vs. the full transcript before.
• "Collapse all" over 115 loaded bubbles: 2.9 ms.
• Paged backwards 6 times to turn 0; anchor updates its count each page and disappears at the start.
• Search expands all groups and highlights matches; zero console errors.
• Small sessions (< 200 turns) render fully with no anchor.
• go test ./..., go vet, golangci-lint clean; screenshots compared against the current bubble design — pixel-identical.

Docs (docs/panel/screens.md, components.md, design brief, panel skill + reviewer agent) updated to describe the windowing and the no-Alpine-per-turn rule.

Note: the in-transcript search only sees loaded turns; server-side transcript search is a natural follow-up if that bites in practice.

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