fix(terminal): page a hollow Codex transcript with PageUp/PageDown from the first wheel event

[aakhter]

This changes the last-resort page-key scroll in _maybePageCliTranscript. That path runs when a wheel or touch gesture is not forwarded to the CLI and the local xterm buffer holds no history (normal buffer, baseY === 0). It sends plain PageUp/PageDown keys so the CLI pages its own transcript.

This is not SGR wheel forwarding. Codex still ignores SGR wheel reports (#227), its wheelForward stays never, and _shouldForwardWheelToApp is unchanged.

1. Codex can use the page-key fallback. Newer Codex draws on tmux's alternate screen, and the full strip hides that from xterm, so a Codex pane has no local scrollback, just like a repaint-mode Claude pane. Scrolling the wheel over it did nothing.

   • Measured with Codex 0.157.1 locally and 0.160 over a remote session: PageUp moves the visible transcript, while SGR wheel reports and local scrolling do nothing.
   • An older inline Codex keeps real local history (baseY > 0), so the guard never fires for it.
   • Which modes may page is a new registry capability, capabilities.transcriptPageKeys (set for claude and codex in stock.ts, an optional boolean in the schema).
   • The server publishes it to the page as window.__codemanTranscriptPageKeys, also in /session/:id windows, so terminal-ui.js holds no CLI id for it. The old mode !== 'claude' check is gone.

2. The first event of a gesture pages straight away. The fallback used to send a key only after half a screen of accumulated travel (19 rows, about 475 px on a 38-row pane), so an ordinary trackpad flick sent nothing.

   • The first page is paid in advance: the travel it skipped is owed back by the rest of the gesture, so the rate stays at one page per half screen.
   • A pause of more than 150 ms, a change of direction or a tab switch starts a new gesture.
   • Sub-row jitter does not page.

Tests. These fail on master and pass with this change:

• test/terminal-scroll-routing.test.ts: Codex pages only while hollow; first-event paging and repayment; gesture reset; tab isolation; modes come only from the injected map.
• test/cli-registry-schema.test.ts: the capability's shape, and exactly claude and codex set in stock.
• test/render-index-html.test.ts: injection in the main page and the solo window.

docs/architecture-invariants.md is updated.

One trade-off: small gestures now send a key where they used to send nothing, for Claude as well. A Claude menu or dialog open at that moment could react to PageUp, which is the caveat this fallback already carries.

[aakhter]

This is a draft only because GitHub currently won't let my account open regular PRs or mark drafts ready (same as #525 and #526). It's ready for review. It stands alone on master and is independent of the tab-rail stack.