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>

src/session.ts

@@ -1973,6 +1973,7 @@ export class Session extends EventEmitter {
           env: buildMuxAttachEnv(cliExportsTruecolor(this.mode)),
         })
       );
+      this._notePtySpawnGeometry(ptyCols, ptyRows);
     } catch (spawnErr) {
       console.error(`[Session] Failed to spawn PTY for ${options.spawnErrLabel}:`, spawnErr);
       this.emit('error', `Failed to attach to mux session: ${spawnErr}`);
@@ -2684,6 +2685,7 @@ export class Session extends EventEmitter {
             env: { ...buildClaudeEnv(this.id), ...(this._envOverrides ?? {}) },
           })
         );
+        this._notePtySpawnGeometry(120, 40);
       } catch (spawnErr) {
         console.error('[Session] Failed to spawn Claude PTY:', spawnErr);
         this._status = 'stopped';
@@ -3306,6 +3308,7 @@ export class Session extends EventEmitter {
             env: buildShellEnv(this.id),
           })
         );
+        this._notePtySpawnGeometry(120, 40);
       } catch (spawnErr) {
         console.error('[Session] Failed to spawn shell PTY:', spawnErr);
         this._status = 'stopped';
@@ -3415,6 +3418,7 @@ export class Session extends EventEmitter {
               env: { ...buildClaudeEnv(this.id), ...(this._envOverrides ?? {}) },
             })
           );
+          this._notePtySpawnGeometry(120, 40);
         } catch (spawnErr) {
           console.error('[Session] Failed to spawn Claude PTY for runPrompt:', spawnErr);
           this.emit(
@@ -4101,6 +4105,17 @@ export class Session extends EventEmitter {
   private _ptyCols = 120;
   private _ptyRows = 40;
 
+  /**
+   * Record the geometry a PTY was just spawned at. A reattached pane keeps the
+   * tmux window's size, not the constructor's 120x40, and without this
+   * `ptyGeometry` reported the old numbers for a live pane and the dedupe in
+   * `resize()` skipped a real resize that happened to match them.
+   */
+  private _notePtySpawnGeometry(cols: number, rows: number): void {
+    this._ptyCols = cols;
+    this._ptyRows = rows;
+  }
+
   /**
    * The geometry the CLI is actually drawing for, or null when nothing is
    * drawing.
@@ -4111,11 +4126,11 @@ export class Session extends EventEmitter {
    * than wrong-sized output, because Claude Code's repaints are computed from
    * the width it was told (issue #464). Both transports report this back.
    *
-   * ⚠️ NULL WITHOUT A PANE, never the field values. `resize()` writes
-   * `_ptyCols`/`_ptyRows` only when `ptyProcess` is set, and nothing seeds them
-   * from the spawn geometry, so a session with a dead pane — or one created
-   * through the API and never started — still holds the constructor defaults
-   * of 120x40. Reporting those made a client adopt a size no process had ever
+   * ⚠️ NULL WITHOUT A PANE, never the field values. The fields are seeded at
+   * spawn (`_notePtySpawnGeometry`) and moved by `resize()`, but a session with
+   * a dead pane (or one created through the API and never started) still
+   * holds the constructor defaults of 120x40, or the size of a pane that is
+   * gone. Reporting those made a client adopt a size no process had ever
    * been told, and on anything narrower than 120 columns it claimed another
    * device owned the pane when none existed. `reconcilePtyGeometry` treats a
    * report with no finite numbers as no evidence, which is the truth here.
@@ -4211,7 +4226,7 @@ export class Session extends EventEmitter {
       if (Date.now() - this._lastDesktopActivityAt < Session.DESKTOP_CLAIM_IDLE_MS) {
         // Declined. The caller is told nothing here on purpose — the decision
         // belongs to the session, not the socket — but the caller MUST report
-        // `ptyCols`/`ptyRows` back afterwards so the asking client can adopt
+        // `ptyGeometry` back afterwards so the asking client can adopt
         // the shape it did not get. Both transports do; see issue #464.
         return;
       }