fix(terminal): merge-time fixes for the silent-failure paths (#431)

- While another device holds the pane width (_paneWidthRefused), a resize
  now asks for the container's width without applying it locally
  (_geometryForResizeRequest: rows follow the container, columns stay at
  the PTY's). Fitting first re-wrapped the whole buffer to the container
  and back on every 30s mobile retry, and throttledResize ran the
  scrollback clear for a resize that brings no redraw. selectSession
  clears the flag, since it belongs to the previous pane. New unit tests
  run the real mixin against a fake terminal and fail without the fix.
- Session seeds _ptyCols/_ptyRows at spawn (_notePtySpawnGeometry), so a
  reattached pane reports its tmux window's real size through ptyGeometry.
- Session.resize's declined-branch comment names ptyGeometry, not the
  deleted ptyCols/ptyRows getters.
- Delete the dead terminalGeometryAgrees() and its window export.
- test/xterm-private-api.test.ts header: it pins the exact locked version,
  so any bump fails, not only a major.
- The main-terminal fit sweep also matches fitAddon?.fit?.(), and
  CLAUDE.md names the modules it actually covers.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/detached-session-pane-sizing.test.ts

@@ -65,6 +65,7 @@ function makeApp(overrides: Record<string, unknown> = {}) {
     // The real chain: sendResize fits, floors and applies through one function
     // now, so the harness must let it (#464).
     syncTerminalGeometry: mixin.syncTerminalGeometry,
+    _geometryForResizeRequest: mixin._geometryForResizeRequest,
     _resizeTerminalTo: mixin._resizeTerminalTo,
     // Real, so a geometry change really does re-check whether the terminal now
     // overflows its container (#464 item 4) — the fake DOM has no container, so