feat(tiles): the grid is cells, and an empty cell can be any cell

Owner feedback 1: "so the empty tab doesnt always have to be the last
one! so I can move freely around and the empty tab can also be tab nr
4 or 3". This replaces the slot refusal of aecada8c.

grid.cells (a session id or null per cell) is now the one source of
truth; grid.ids is a getter deriving the tiles in reading order, so
everything that only wants the tiled sessions (focus neighbour,
cycling, the load queue's order, the picker, closeSession) is
unchanged. The shape still comes from the tile count and the cap
counts tiles, never empty cells.

- A tile dragged onto an empty cell moves there and leaves its own
  cell empty, nothing else moving (_moveTileToCell, through
  _reorderTiles: no remount, reconnect or reload; only a tile whose
  cell size changed fits). A tiled session's tab does the same; a tab
  of a session not tiled yet joins in the cell it is dropped on. Each
  slot knows its cell and reads "Drop a tab or a tile here".
- Move Tile goes to the adjacent cell: into it when empty, a swap when
  a tile is there (tileCellInDirection).
- Removing a tile leaves its cell empty; adding one takes the first
  empty cell. A shape change goes through fitTileCells: each tile keeps
  its row and column when all fit (2x2 growing to 3x2), else the tiles
  pack in reading order.
- Focus never lands on an empty cell: Alt+Shift+Arrows run over the
  cells, Ctrl+Tab and Alt+[ ] over the tiles.
- codeman:tile-grid stays ids only: its ids are the cells with null for
  an empty one. A reload brings the holes back when the shape is the
  same (a session gone since leaves its cell empty), another shape
  packs, the old packed format reads unchanged, and a followed
  #session= link keeps the holes.

Docs: the spec's as-built bullet (rewritten in place), the wiki's Tile
Grid page and Keyboard Shortcuts, the invariants and CLAUDE.md.

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

docs/tile-grid-plan.md

@@ -21,8 +21,8 @@ or settled a question the spec left open. The invariants as built are in
   the setting off the toggle chord is inert. A grid opened another
   way (Ctrl/Cmd+click, a dropped tab, "Open group as tiles") keeps all its chords.
 - **Dividers are grid tracks.** Each gap between columns and rows is its own 6px track (the
-  grid gap is 0) and tiles are placed explicitly in reading order, which is also what the
-  empty-slot drop targets need. Fractions reset when the column or row count changes.
+  grid gap is 0) and every tile and every empty slot is placed explicitly in its cell
+  (`grid.cells`, see "Tiles move"). Fractions reset when the column or row count changes.
 - **Zoom follows tmux.** Moving focus to another tile restores the grid; an automatic zoom
   (window too small for the minimum tile) follows focus instead.
 - **Tile loads are bounded** (`boundedLoad`), carry a fetch deadline covering the body (Pane
@@ -68,28 +68,46 @@ or settled a question the spec left open. The invariants as built are in
   batches, so lifecycle and hook events are unaffected, and leaving the grid
   re-subscribes the shown session.
 - **Tiles move** (owner request: "give me the option to move the tiles around"; not a
-  numbered decision). A tile's header, its free area (not the buttons, not the rename
-  input), drags it onto another tile and the two trade places. An empty slot refuses a tile,
-  its header drag and its tab alike (owner's answer, "dont move the tile": a slot is always
-  the last cell, so a move there shifted every tile after it); a session not tiled yet still
-  joins there. It is a native drag through the tab drop targets (capture phase, stopped
-  before xterm), carrying a type of its own and never text, and it is not `draggedTabId`, so
-  neither a text field nor the tab strip takes it; Escape or a drop anywhere else cancels
-  with nothing changed, focus included: the header focuses its tile on click, not on press
-  (owner's answer: best practice; the body keeps press-to-focus, so focus moves before a
-  press reaches xterm). `Ctrl+Shift+Arrows` (Move Tile Left/Right/Up/Down, registry,
-  rebindable) swap the focused tile with the neighbour the focus chords pick, and focus
-  stays on it. Every move, a tiled tab's drop included, goes through `_reorderTiles`: no
-  remount, reconnect or reload; divider sizes belong to the cells, so only a tile whose cell
-  size changed fits (one PTY resize, #464). Moving is off while a tile is zoomed (the chords
-  still apply there, as a no-op, so their keys never reach the CLI; a tiled tab dropped on
-  the zoomed tile is refused too, as the owner confirmed) and with a single tile. Both arrow
-  chord families, focus and move, skip a text field, where shifted arrows select (owner's
-  answer: best practice). Default keys: every other two-modifier arrow chord is taken
-  (Ctrl+Alt switches workspaces, Ctrl+Alt+Shift moves a window to another workspace in
-  GNOME, Alt is back/forward, Alt+Shift focuses tiles); Ctrl+Shift+Arrows is unclaimed by
-  the browsers, GNOME, KDE, macOS and Claude Code, and costs only a terminal editor's word
-  selection inside a tile while the grid is open.
+  numbered decision). The grid is CELLS, not a packed list (owner: "the empty tab doesnt
+  always have to be the last one ... it can also be tab nr 4 or 3"): `grid.cells` holds a
+  session id or `null` per cell and is the one source of truth, `grid.ids` the tiles in
+  reading order derived from it. The shape still comes from the tile count (the layout
+  table), the cap counts tiles, never empty cells, and an empty cell can be any cell (in
+  practice one at most: a 3x2 holds 5 or 6 tiles, a 2x2 3 or 4). A tile's header, its free
+  area (not the buttons, not the rename input), drags it: onto another tile the two trade
+  places, onto an empty cell it moves there and leaves its own cell empty, nothing else
+  moving; a tiled session's tab does the same, and a tab of a session not tiled yet joins in
+  the cell it is dropped on. It is a native drag through the tab drop targets (capture
+  phase, stopped before xterm), carrying a type of its own and never text, and it is not
+  `draggedTabId`, so neither a text field nor the tab strip takes it; Escape or a drop
+  anywhere else cancels with nothing changed, focus included: the header focuses its tile on
+  click, not on press (owner's answer: best practice; the body keeps press-to-focus, so
+  focus moves before a press reaches xterm). `Ctrl+Shift+Arrows` (Move Tile
+  Left/Right/Up/Down, registry, rebindable) move the focused tile to the adjacent cell: into
+  it when empty, trading places when a tile is there, never jumping a cell; focus stays on
+  it. Every move goes through `_reorderTiles`: no remount, reconnect or reload; divider
+  sizes belong to the cells, so only a tile whose cell size changed fits (one PTY resize,
+  #464). Removing a tile leaves its cell empty where it was, and adding one takes the first
+  empty cell (or the one a tab was dropped on), while the shape stays; a shape change
+  (`fitTileCells`, constants.js) keeps each tile's row and column when all fit and otherwise
+  packs the tiles in reading order, which differs from plain packing only when a 2x2 grows
+  to a 3x2 (the four tiles stay put). Focus never lands on an empty cell: Alt+Shift+Arrows
+  go along the row past one, or to the nearest row with a tile (the same column, else the
+  nearest), and Ctrl+Tab and Alt+[ / ] cycle the tiles only. `codeman:tile-grid` stays ids
+  only: its `ids` are the cells, `null` for an empty one (a build before cells drops the
+  nulls and reads them packed); a reload brings the holes back when the shape is the same, a
+  session gone by then leaves its cell empty, another shape packs, and the old packed format
+  reads unchanged. A fresh grid (the picker's Open, Ctrl/Cmd+click with the grid closed,
+  "Open group as tiles") opens packed; only the toggle and the page-load restore bring holes
+  back. Moving is off while a tile is zoomed (the chords still apply there, as a no-op, so
+  their keys never reach the CLI; a tiled tab dropped on the zoomed tile is refused too, as
+  the owner confirmed) and with a single tile. Both arrow chord families, focus and move,
+  skip a text field, where shifted arrows select (owner's answer: best practice). Default
+  keys: every other two-modifier arrow chord is taken (Ctrl+Alt switches workspaces,
+  Ctrl+Alt+Shift moves a window to another workspace in GNOME, Alt is back/forward,
+  Alt+Shift focuses tiles); Ctrl+Shift+Arrows is unclaimed by the browsers, GNOME, KDE,
+  macOS and Claude Code, and costs only a terminal editor's word selection inside a tile
+  while the grid is open.
 - **A tile that joins before its pane exists resends its size when the pid appears**
   (`TerminalTile.paneStarted()`): the server drops a resize for a session with no PTY and
   spawns at 120x40, and Run's own resize measures the parked main terminal. Applying a