BlockEditor selects across blocks: drag or Shift-press, then type, paste, delete, cut, copy as Markdown or break, one undo step (#7)

AGENTS.md

@@ -177,6 +177,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - A component that fills its container takes no width on its root: a block or a flex column stretches it, in a flex row the host gives it `flex_1`, and as a window's root it fills the window, since gpui sets a window root's auto sides to the window's size. A percent width ignores the root's own margins, so they push it past its container, and in a flex row it takes the whole row as its basis and leaves a `flex_1().min_w_0()` neighbor nothing. A root centered under a cap keeps `w_full`, since a flex column stretches no child with an auto side margin, and `mx_auto` needs a width to fill (`layout::Container`). So does a box that keeps a ratio: a block follows its ratio from a stretch, but a flex or grid box only from a set width, and callers may make one flex (`layout::AspectRatio`, and `primitives::framed` under VideoPlayer, CameraPreview, ModelViewer, ParticipantTile and InpaintCanvas). `Button::full_width` is its host asking, since a button takes no style. Each chapter's test holds its filling roots at the full width of a flex column in a padded block through `layout::tests::narrow_width`, which reads a root by its `debug_selector`.
 - A field's `Highlight` sets color, wash, weight, slant and strike per span; measuring and painting share one set of runs, so wrapped heights match.
 - A block editor keeps one undo for the document: text and structure together, as snapshots through `forms::History`; it takes Undo and Redo before its fields do.
+- A selection across a BlockEditor's fields lives in the editor (`blocks::span`): a press in a field drags into others, Shift-press reaches from the caret's field. The pressed field selects its own part; the editor sets the other end's and washes blocks between. The editor watches the focused end through `TextInput::edits`, which counts every edit, text changed or not: an edit there, once an input method commits, is rebuilt from the last snapshot as head, typed and tail in one undo step; a caret move or lost focus lets the span go, since gpui runs bindings before key listeners. Backspace, Delete, Cut and Enter take the span before the field acts; copy writes it as Markdown.
 - gpui calls focus listeners only while the window is active. A test that needs focus events activates its window with `window.activate_window()`.
 - A terminal takes Tab and Shift-Tab through bindings under `ElyTerminal`, ahead of focus moves; other keys go through `terminal::keys` in a key listener, and typed text through its input handler.
 - A shell in the gallery starts without startup files and with a fixed prompt, so captures show no user or host.