test sweep: cover BufferLine + CellAnchor (T-91)

Adds test/terminal/buffer/line_test.dart — 51 unit tests covering
BufferLine and CellAnchor, hitting every reachable line in
lib/src/terminal/src/core/buffer/line.dart (192 / 192).

Coverage delta:
- line.dart: 0 / 194 → 192 / 192 (file shrank by two lines after
  the prior commit's iteration fix folded two for-loop heads into
  for-each-toList).
- Total project: 36.39% → 39.12%.
- pubspec.yaml `coverage_floor:` bumped 36 → 39 in lockstep.

Highlights:
- All packed-cell encodings (foreground/background/attrs/content
  channels, codepoint+width packing, CellData round-trips).
- `eraseRange` wide-char neighbor extension on both ends.
- `removeCells` / `insertCells` shift logic, anchor reposition, and
  the wide-tail-erase branch (insertCells case where the post-shift
  last cell carries a wide marker).
- `resize` exercising the [64, 256) capacity-doubling branch and
  the >=256 +32 branch separately.
- `getTrimmedLength` cols-clamp behaviour for null/over-capacity.
- `getText` skip-trailing-wide-char branch.
- `CellAnchor` lifecycle: detached construction, `reposition`,
  `reparent` (both detached→attached and between owners), `dispose`,
  attached y/offset via a real IndexAwareCircularBuffer.

Also cleans up five `unrelated_type_equality_checks` analyze infos
in test/terminal/buffer/range_test.dart by typing the RHS as Object
when intentionally probing the type-mismatch branch of operator==.

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

pubspec.yaml

@@ -18,7 +18,7 @@ repository: https://github.com/postmeridiem/clide
 
 # Pre-push line-coverage floor. Ratchets up only — see D-66.
 # Reading: `awk -F: '/^coverage_floor:/ {gsub(/ /,"",$2); print $2}' pubspec.yaml`.
-coverage_floor: 36
+coverage_floor: 39
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads