fix(tiles): popping out a tiled session keeps the grid's count

A session popped out while the grid was open (its pop-out button, or
another dashboard tab announcing the pop-out over the window channel) left
the grid through _markDetached, which removed its tile as if the user had
closed it: the stored count dropped to five and its cell stayed a hole the
ranking never filled, so after the window re-docked Tiles off and on showed
five tiles and an empty cell. Popping out with the grid closed, or a reload
reconcile, already kept the count and refilled the cell.

_markDetached now removes the tile as `gone` (it left by itself), the same
as a deleted session or a refused socket, so the count stays and the next
activation fills the cell from the ranking.

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

docs/architecture-invariants.md

@@ -839,7 +839,7 @@ Tests: `test/tab-rail-search.test.ts` (gate) and `test/tab-rail-search.browser.t
 
 ⚠️ **Attach.** A tile whose session has no PTY (`pid === null`) or whose socket closed because it exited (4009) shows an Attach overlay (absolute, the body keeps its size): `POST /interactive` (or `/shell`) with NO body, one in flight per session, a tripped PTY-exit breaker only through the same confirm as the single view, then the tile is remounted (a stopped socket cannot reconnect). The routes report a refusal in the ENVELOPE of a 200, so the response body is read, not `res.ok`. An agent that exited in a live pane (`paneExit`) cannot be started again in place (both routes refuse while the pane's tmux client runs): its tile shows the exit and points at Close session.
 
-⚠️ **Persistence and joining.** `codeman:tile-grid` (localStorage, per browser, never sent to the server) holds `{ v: 1, open, ids, count, focused, zoomed, colFr, rowFr }`, session ids and layout only, never content, written on every change (move, divider pointer-up, add, remove, count pick, focus, zoom); its `ids` are the CELLS, `null` for an empty one, and `sanitizeTileGridState` returns them as `cells` (a dropped id a hole, never a shift) beside the packed `ids` every list consumer wants, plus `freed` (cells whose session went away since) and `count`. ⚠️ `count` is how many tiles the user's own last change left: `removeTile(..., { gone: true })` (the `_onSessionDeleted` wrapper, `_reconcileTileGrid`, `_onTileExit`) does NOT lower it, so the next activation refills that cell, while a removal by hand does; it is derived (the sessions the cells name) for a value written before it, and the format stays `v: 1` so an older build still reads a newer value. A restore keeps the cells when their shape matches the current one, else `reformTileCells` (positions kept when all fit, else packed). ⚠️ Every close keeps the grid as `open: false` (`closeTileGrid({ keepStored: true })` from every caller: the toggle, a non-tiled pick, `leaveTiles`, Home, the width gate, the last tile leaving, "Open group as tiles", kill-all); `_closeStoredTileGrid` (a `#session=` link on load) flips `open` on the RAW stored value, so a gone id still frees its cell; `_openStoredTileGrid` holds `_persistTileGrid` (`_tilePersistHold`) until the grid is back, so `openTileGrid`'s packed intermediate layout is never written over it. Only when none of its sessions survive does activation rank from scratch. A solo window never reads or writes it, an automatic zoom is not stored. Sessions created by THIS tab's Run join the open grid (`_joinTileGridFromRun`, called from session-ui.js `_ensureCreatedSessionVisible`; a wrapper from tile-grid.js would be overwritten by session-ui.js's later `Object.assign`); sessions created elsewhere arrive only by `session:created` and never join. ⚠️ A tile that joins that way connects BEFORE Run starts its pane: its resize reaches a session with no PTY, which `Session.resize()` only records (`_lastDesktopDims`) while the spawn uses a fixed 120x40, and Run's own resize step measures the parked main terminal (`display: none`, so `proposeDimensions()` is NaN and the step is skipped). Measured live: a 97x17 tile over a 120x40 pane. So `_renderTileChrome` remembers each tile's last-seen pid and calls `tile.paneStarted()` when it appears or changes (keyed on the sessions map, so a `handleInit` after an SSE drop counts too); `paneStarted()` forgets the sent size and sends it, and a hidden tile (a zoomed neighbour) sends nothing but keeps the size forgotten, so its next `fit()` sends it. A plain `fit({ force: true })` would lose that case. Tests: `test/tile-grid-*.test.ts` over the shared vm harness `test/mocks/tile-grid-vm.ts`.
+⚠️ **Persistence and joining.** `codeman:tile-grid` (localStorage, per browser, never sent to the server) holds `{ v: 1, open, ids, count, focused, zoomed, colFr, rowFr }`, session ids and layout only, never content, written on every change (move, divider pointer-up, add, remove, count pick, focus, zoom); its `ids` are the CELLS, `null` for an empty one, and `sanitizeTileGridState` returns them as `cells` (a dropped id a hole, never a shift) beside the packed `ids` every list consumer wants, plus `freed` (cells whose session went away since) and `count`. ⚠️ `count` is how many tiles the user's own last change left: `removeTile(..., { gone: true })` (the `_onSessionDeleted` wrapper, `_reconcileTileGrid`, `_onTileExit`, and app.js `_markDetached` for a session popped out while the grid is open) does NOT lower it, so the next activation refills that cell, while a removal by hand does; it is derived (the sessions the cells name) for a value written before it, and the format stays `v: 1` so an older build still reads a newer value. A restore keeps the cells when their shape matches the current one, else `reformTileCells` (positions kept when all fit, else packed). ⚠️ Every close keeps the grid as `open: false` (`closeTileGrid({ keepStored: true })` from every caller: the toggle, a non-tiled pick, `leaveTiles`, Home, the width gate, the last tile leaving, "Open group as tiles", kill-all); `_closeStoredTileGrid` (a `#session=` link on load) flips `open` on the RAW stored value, so a gone id still frees its cell; `_openStoredTileGrid` holds `_persistTileGrid` (`_tilePersistHold`) until the grid is back, so `openTileGrid`'s packed intermediate layout is never written over it. Only when none of its sessions survive does activation rank from scratch. A solo window never reads or writes it, an automatic zoom is not stored. Sessions created by THIS tab's Run join the open grid (`_joinTileGridFromRun`, called from session-ui.js `_ensureCreatedSessionVisible`; a wrapper from tile-grid.js would be overwritten by session-ui.js's later `Object.assign`); sessions created elsewhere arrive only by `session:created` and never join. ⚠️ A tile that joins that way connects BEFORE Run starts its pane: its resize reaches a session with no PTY, which `Session.resize()` only records (`_lastDesktopDims`) while the spawn uses a fixed 120x40, and Run's own resize step measures the parked main terminal (`display: none`, so `proposeDimensions()` is NaN and the step is skipped). Measured live: a 97x17 tile over a 120x40 pane. So `_renderTileChrome` remembers each tile's last-seen pid and calls `tile.paneStarted()` when it appears or changes (keyed on the sessions map, so a `handleInit` after an SSE drop counts too); `paneStarted()` forgets the sent size and sends it, and a hidden tile (a zoomed neighbour) sends nothing but keeps the size forgotten, so its next `fit()` sends it. A plain `fit({ force: true })` would lose that case. Tests: `test/tile-grid-*.test.ts` over the shared vm harness `test/mocks/tile-grid-vm.ts`.
 
 ### Gesture control: the setting