feat(tiles): the grid holds at most 6 tiles (owner decision 7)

Six was tested smooth on the owner's desktop; nine missed the headless
frame bar and is untested on real hardware. TILE_GRID_MAX (constants.js)
is now 6 and stays the one cap every limit reads; the layout table gets
its own bound, TILE_LAYOUT_MAX = 9, so the 7 to 9 layouts keep working
(unreachable) and going back to nine is that one line.

Every way in stops at the cap: opening, addTile, a tile's +, a session
Run makes, Ctrl/Cmd+click, the picker, "Open group as tiles", and a stored
grid with more ids (it comes back as its first six, focus kept only if it
survives, a dropped zoom cleared, row fractions that no longer match the
3x2 reset). The limits now go through one helper, _tileGridLimit(), whose
texts say which limit binds: "Up to 6 tiles" / "The grid holds at most 6
tiles" when it is the cap, "This window fits N" when it is the window.

Docs: decision 7 and an as-built entry in the spec, CLAUDE.md, the
invariants, the wiki's Tile Grid and Dashboard pages.

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

docs/architecture-invariants.md

@@ -797,7 +797,7 @@ Further detail: with many sessions the horizontal strip stops being scannable, w
 
 ### Tile grid
 
-**Tile grid** (`showTileGridButton`, header Tiles button + `Ctrl+Shift+G`, default OFF, per-device: in `displayKeys`, stripped from the settings PUT, not in `SettingsUpdateSchema`; desktop-only at 1180px by a JS width check with a live media listener plus a `@media (max-width: 1179px)` backstop, never in a solo window). 1 to 9 live sessions side by side in one window, each a `TerminalTile` (terminal-tile.js), orchestrated by tile-grid.js (load order 7.6, `CodemanApp.prototype` methods like the split's). Pure helpers live in constants.js as `window.CodemanTileGrid`: `computeTileLayout` (1x1, 2x1, 3x1 on a grid area at least 1800px wide else 2x2, 2x2, 3x2, 3x3; `fits` against a 480x240 minimum tile), `tileGridCapacity`, `sanitizeTileGridState`, `buildTilePickerSessions`, `dragTrackFractions`, `tileNeighbor`, `tileInDirection`, `cycleTile`, `TILE_SCROLLBACK` (10,000). The grid is a `<section class="tile-grid">` SIBLING of `.terminal-wrap`, swapped in by `.main.tiles-active` (no reparenting); `.main.webview-active .tile-grid` hides it like the single view.
+**Tile grid** (`showTileGridButton`, header Tiles button + `Ctrl+Shift+G`, default OFF, per-device: in `displayKeys`, stripped from the settings PUT, not in `SettingsUpdateSchema`; desktop-only at 1180px by a JS width check with a live media listener plus a `@media (max-width: 1179px)` backstop, never in a solo window). 1 to 6 live sessions side by side in one window, each a `TerminalTile` (terminal-tile.js), orchestrated by tile-grid.js (load order 7.6, `CodemanApp.prototype` methods like the split's). Pure helpers live in constants.js as `window.CodemanTileGrid`: `computeTileLayout` (1x1, 2x1, 3x1 on a grid area at least 1800px wide else 2x2, 2x2, 3x2, and 3x3 up to `TILE_LAYOUT_MAX` (9), unreachable today; `fits` against a 480x240 minimum tile), `tileGridCapacity` (never more than `TILE_GRID_MAX`), `sanitizeTileGridState` (truncates a stored grid to the cap, keeps focus only if it survives), `buildTilePickerSessions`, `dragTrackFractions`, `tileNeighbor`, `tileInDirection`, `cycleTile`, `TILE_SCROLLBACK` (10,000), and `TILE_GRID_MAX` (6), the ONE cap (owner decision 7: six tested smooth on a real desktop, nine missed the headless frame bar). ⚠️ Every limit reads the cap through `_tileGridLimit()` (tile-grid.js: the window's capacity, at most the cap), never a literal, and its texts say which binds ("at most 6 tiles" vs "what this window fits"). The grid is a `<section class="tile-grid">` SIBLING of `.terminal-wrap`, swapped in by `.main.tiles-active` (no reparenting); `.main.webview-active .tile-grid` hides it like the single view.
 
 ⚠️ **The main terminal is parked while the grid is open.** Opening runs `_cleanupPreviousSession()` ONCE (its snapshot is right at that moment, and it closes the main socket), and `activeSessionId` is always the FOCUSED tile's session, so everything keyed on it (files panel, git status, respawn and Ralph panels, subagent windows, voice, image paste, the tab highlight) follows focus. With the main socket closed `_wsReady` is false, so every SSE terminal handler would write the focused tile's output into the hidden xterm: `_tilesOwnTerminal()` turns `_onSessionTerminal`, `_onSessionClearTerminal`, `_onSessionNeedsRefresh` (returns `false`, which the drop recovery reads), `_scheduleDroppedOutputRecovery`, the `writeln` in `_onSessionCompletion`/`_onSessionError`, `sendResize`, `throttledResize`'s fit and `_maybeRefetchFullHistory` into no-ops; `retryConnection` and `handleInit` re-arm the TILES' sockets instead of the main one (`handleInit` keeps live tiles, drops dead ones through `_reconcileTileGrid`, and re-selects only when the focused tile is gone, so an SSE blip never hides an open web tab). ⚠️ The WebGL long-task observer watches the WHOLE page: it counts nothing while tiles own the terminal, or tile renders would write the sticky 7-day WebGL disable. The header connection dot reads the tile sockets (`_tileGridSocketState`; a tile stopped for good does not count). `_focusedPane()` answers with the focused tile even when DOM focus left every terminal, and `_forEachTile` reaches every grid tile (`{ grid: false }` skips them where tiles keep their own font size). Leaving the grid destroys every tile, resets `_lastResizeDims`, invalidates the main terminal's cached content for EVERY tiled id (`_xtermSnapshots`, `codeman-xs-<id>`, `terminalBufferCache`: written before the grid opened, and selectSession paints a snapshot as its first frame) and replays the focused session through `selectSession(id, { forceReload: true, auto: true })`.