fix(#4343): default transcript virtualization OFF (experimental/opt-in) + force-off migration

Virtualization (#500/#4214) caused a scroll-up flicker on long sessions with
tall tool-call rows (variable-height anchor oscillation), distinct from the
stream-completion jump fixed in #4325. Per maintainer decision, flip the
'Virtualize long transcripts' preference to experimental/opt-IN (default OFF)
for everyone until the scroll behavior is reliable.

- api/config.py: virtualize_transcript default True->False; add
  virtualize_transcript_optin marker (both bool-allowlisted); load_settings
  force-off migration — a stored true WITHOUT the post-flip opt-in marker is
  reset to off, so 100% of existing users land on off; an explicit opt-in
  (marker present) is honored.
- boot.js / panels.js: default-off semantics (===true), persist the opt-in
  marker on enable, checkbox not pre-checked.
- index.html / i18n.js (en): relabel experimental + flicker warning.
- Tests: rewrite the #4325 toggle assertions for the opt-in contract + 4 new
  migration tests (unset/stale-true/explicit-optin/marker-without-true).

Phase B (root-cause scroll-stability) tracked separately. Thanks @b3nw.

CHANGELOG.md

@@ -3,6 +3,12 @@
 
 ## [Unreleased]
 
+## [v0.51.470] — 2026-06-17 — Release QE (transcript virtualization now experimental / off by default)
+
+### Changed
+
+- **Transcript virtualization is now experimental and off by default (#4343).** The windowed rendering of very long transcripts (#500/#4214) caused a scroll-up flicker on long sessions with large tool-call blocks — the viewport "flicks back and forth" while scrolling up (a variable-height virtual-scroll anchor oscillation, distinct from the stream-completion jump fixed in #4325). Until the scroll behavior can be made reliable, the "Virtualize long transcripts" preference now defaults **off** for everyone — including anyone who had it on during v0.51.469 — and is relabeled experimental/opt-in. By default the full transcript renders at once (no flicker, and the browser's Find/Ctrl+F matches the whole conversation). You can opt back in under Settings → Preferences. Thanks @b3nw for the report and A/B confirmation. (Root-cause scroll-stability work tracked separately.)
+
 ## [v0.51.469] — 2026-06-17 — Release QD (transcript virtualization fix + opt-out)
 
 ### Fixed