#36188 Flicker log: off in gallery pages, and the ring stays under its bound after each write

The two follow-ups from the review of #36133 (the always-on flicker log behind Leo's report, 2026-11-04: "a flicker across nearly the full screen").

• F1, gallery pages: the gallery's stand-in for the app's pane handler (gallery/frame/agentPane.ts) marks itself standIn, and startFlickerLog does not start on it. Gallery call logs get no diag.flicker events, and gallery pages run no detector observers.
• F2, ring bound: AgentPageFlickerLog rotates when a write would take the file past 256 KB, not after it, so neither half of the ring grows past the bound. The rollover test checks both files.

🤖 Generated with Claude Code

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Summary by cubic

Turns off the flicker log in gallery pages and makes the ring rotate before a write would exceed the 256 KB bound.

• The gallery's stand-in for the app's WebKit handler now marks itself standIn, so startFlickerLog doesn't start there and gallery call logs emit no diag.flicker events.
• AgentPageFlickerLog rotates when a write would take the file past 256 KB, not after it, so neither half of the ring grows past the bound.