emrg: GUI scroll capture fix — back-to-bottom + history load respond to session-view scrolls

[argszero]

Summary

Fixes a silent regression from #634 (per-session .session-view scroll containers) that also affected the v0.2 workspace views (#762).

Problem

#workspace is overflow: hidden and never scrolls itself — the actual scrollers are the .session-view.active children. Both scroll listeners were attached to #workspace in the bubble phase, but native scroll events do not bubble, so:

• the back-to-bottom button never appeared in the real GUI (tests dispatched scroll directly on the wrapper, masking the bug)
• scroll-to-top → load older history never fired (same cause)
• returning from a panel view (v0.2) always jumped to the bottom, violating the rant acceptance "scroll position preserved" (browser-tab semantics)

Fix

• attach both #workspace scroll listeners with capture: true so child .session-view scrolls reach them
• updateBackToBottomState() / activeScrollEl() helpers read metrics from the active .session-view (not the non-scrolling #workspace)
• back-to-bottom click scrolls the active .session-view
• activateSessionView only scrolls to bottom for newly-created containers; existing containers preserve their scroll position (panel round-trip / session switch no longer jump)
• setWorkspaceChrome hides back-to-bottom in panel views, restores per position when returning to session view

Verification

• +1 renderer.smoke test: back-to-bottom follows session-view scroll (capture), panel-view hide, position restore on return — GUI 236→237
• pytest 801 green · node --check clean · Agent.md count synced (doc-count guard 3/3)

[argszero]

✅ LGTM — self-review (1/3). Local: GUI 237/237 (new scroll-capture regression test), pytest 801, node --check clean, doc-count guard 3/3. CI test + test-windows PASS. Diff verified: capture:true on #workspace scroll listeners; metrics read from active .session-view via activeScrollEl; activateSessionView preserves scroll for existing containers (browser-tab semantics per rant acceptance); panel view hides back-to-bottom + restores on return.

[argszero]

✅ LGTM — self-review (1/3). Verified locally: GUI tests 237/237 (new regression test covers scroll-up → back-to-bottom shows + autoScroll=false, click → scrollTop=scrollHeight + button hides, panel view → button force-hidden, return to session → button restored per scroll position), pytest 801, doc-count guard green (Agent.md 236→237 sync). CI 31695553181 test + test-windows PASS, MERGEABLE.

Root-cause audit confirms correctness:

• Native scroll events do NOT bubble → previous bubble-phase listener on #workspace never fired (workspace is overflow:hidden; actual scrollers are .session-view children) — real-GUI silent regression since #634, also hit v0.2 workspace views (#762).
• Fix uses capture: true on the #workspace listeners (both back-to-bottom and history-load) → captures child .session-view scrolls. Correct mechanism.
• activeScrollEl() resolves the real scroller (active session view, falling back to first .active child / workspace); back-to-bottom click scrolls the resolved element.
• activateSessionView scrolls to bottom ONLY for newly-created containers (existed check) — preserves scroll position on panel round-trip / session switch, satisfying the rant 18:55:09 acceptance.
• setWorkspaceChrome('panel') force-hides back-to-bottom; 'sessions' restores via updateBackToBottomState().

No issues found; awaiting 2 more ✅ from independent cycles.

[pm25coder]

I tested this PR end-to-end (head b3b7246) and found the fix is correct and complete:

[argszero]

✅ LGTM — cycle (3/3). Fresh verification on head b3b7246: pytest 801 passed, GUI 237/237 passed, node --check clean. Diff reviewed: capture:true scroll listeners reach child .session-view (scroll doesn't bubble); metrics via activeScrollEl from the scrolling container; activateSessionView preserves scroll position for existing containers (browser-tab semantics per rant 18:55:09 acceptance); panel view hides back-to-bottom, restores per position on return. CI test + test-windows PASS. Merging.