docs(tabs): drag stays on during a rail search (#580)

Owner decision: searching for a tab and dragging it into a group is a useful
flow, so the drag-off guard is reverted and the docs and changeset say drag
works during a search.

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

docs/architecture-invariants.md

@@ -802,7 +802,7 @@ Further detail: with many sessions the horizontal strip stops being scannable, w
 - **Collapsed groups.** A collapsed group's rows are not in the DOM, so a class cannot reveal them: the projection opens every group while the rail search runs (see Owner tab layouts), and `setTabRailSearch()` re-renders only when that changes the structure (`_isTabGroupStructureStale()`); every other keystroke only re-applies classes.
 - **Connectors.** Lineage lines and the subagent/ultracode connectors are anchored to row positions, and a keystroke re-renders nothing, so `_applyTabListFilter()` calls `updateConnectionLines()` whenever it actually showed or hid something (a row, a section, the state headings via `tabs-filtering`, the empty note). An unchanged re-apply at a render tail calls nothing, which keeps the incremental path's lineage gate meaningful. ⚠️ A hidden row still answers `getBoundingClientRect()` with an all-zero rect, which is truthy, so every floating window that anchors to its parent tab (the subagent and ultracode connectors' shared `tab:<id>` cache, both spawn positions, the ultracode genie) finds it through `_paintedSessionTab()` (app.js: the row only while `getClientRects()` is non-empty). An unpainted parent draws no connector, spawns where a window without a tab would, and tears down without the genie; otherwise all three started at the viewport's top-left corner. Lineage needs no guard, since `computeTree` drops zero-size rects.
 - **Escape.** The global key handler runs in the CAPTURE phase on `document`, before the box's inline `onkeydown`, so a `stopPropagation()` there comes too late: the global Escape branch claims an Escape whose target is `#tabRailSearch` while the box holds text (beside the group menu, the grouped-rail drag and the Tiles count menu) and routes it to `handleTabRailSearchKeydown()`. Without that, clearing the search also collapsed the Monitor and Subagents panels. An empty box leaves Escape to the global handler.
-- **No drag while searching** (owner decision): a drop is saved for every device, relative to rows the search hides. Both rail drags refuse in their start handlers (`_onTabLayoutPointerDown`, the flat rail's `dragstart`), never by flipping `draggable`, which only a render would restore.
+- **Drag stays on while searching** (owner decision): searching for a tab and dragging it into a group is a useful flow. A drop uses the same anchor logic relative to the visible rows, so a row can land between visible neighbours with hidden rows on either side.
 - **In memory only, cleared off the rail.** The text is never persisted or sent. `applyTabOrientation()` calls `_resetTabRailSearch()` before its render when the list leaves the vertical rail, the same reason leaving sidebar mode clears `_sidebarFilter`.
 
 Tests: `test/tab-rail-search.test.ts` (gate) and `test/tab-rail-search.browser.test.ts` (browser suite, not in the gate; it installs the real `setupEventListeners()` so the capture-before-inline Escape order is the shipped one).