fix dangling tail anchors in reflow on partially-filled lines (T-92)

`_LineReflow._addPart`'s post-loop block reparents anchors past the
source line's trimmed content onto whatever `_builder._result` was
active at that moment. When no further content lands in the builder
(non-wrapped lines, or the last logical line of a wrapped run),
`finish()` was emitting only when `_builder.isNotEmpty` — leaving
the empty result line with the reparented anchor unappended. The
anchor then pointed to a `BufferLine` that the reflow output never
included, `lines.replaceWith(reflowResult)` discarded it, and
`CellAnchor.attached` returned false. The selection controller's
`extent.attached` null-check then dropped the selection silently
on resize.

The fix adds a `_LineBuilder.hasAnchors` getter and uses it in
`finish()` so the builder line is also emitted when it's carrying
an anchor — even when otherwise empty. Trade-off: an extra trailing
line in the reflow output when (and only when) a tail anchor would
have dangled. `Buffer.resize` already pads the result to `newHeight`
afterward, so for the common case (resize fits inside view height)
the total ring length is unchanged; only when the result already
meets / exceeds `newHeight` does the buffer grow by one. Acceptable
in exchange for selections surviving a width change.

User-visible trigger paths:
- `SelectAllTextIntent` (Ctrl+A) creates an end anchor at
  `x = viewWidth` on the last buffer line — exactly the past-
  trimmed-length position. Resizing narrower while the selection
  was active dropped it.
- Mouse drag selections past the end of a partially-filled line
  hit the same shape.

Tests:
- The pre-existing `reflow anchors on the source line tail (past
  trimmedLength) get reparented` test was originally written to
  document the buggy behaviour ("anchor moves off the source onto
  a dangling builder line"). Updated to assert the post-fix
  contract: `out.contains(tail.line)` is true.
- New `SelectAllTextIntent-shaped end anchor survives shrink`
  regression test that mirrors the actual production trigger
  (anchor at `x = viewWidth` on a partially-filled line, narrower
  reflow).

reflow.dart 71/71 → 72/72 (the new getter is a one-liner). Project
coverage 54.62% unchanged within rounding.

Co-Authored-By: Claude <noreply@anthropic.com>

CHANGELOG.md

@@ -47,6 +47,15 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Fixed
 
+- Terminal selections no longer silently disappear when the terminal is
+  resized narrower. Reflow's tail-anchor handler used to reparent
+  anchors past the source's trimmed-content range onto a builder line
+  that was never emitted, leaving them detached from the visible
+  buffer; the selection controller's `extent.attached` check then
+  returned null and the highlight vanished. Common trigger paths:
+  Ctrl+A (select-all) followed by a width change, and mouse drag
+  selections that extended past the end of a partially-filled line
+  (T-92).
 - `BufferLine.removeCells` / `insertCells` / `dispose` no longer skip
   anchors due to concurrent list modification during iteration —
   surfaced by unit tests added under T-91. Anchors disposed inside