test sweep: cover Buffer (T-91)

Adds test/terminal/buffer/buffer_test.dart — 67 unit tests against
the Buffer orchestrator on top of BufferLine. Drives a fake
TerminalState through writes, cursor moves, scroll regions,
erase commands, line insert/delete, resize (with and without
reflow), word-boundary lookup, getText, and the toString debug
dump.

Coverage delta:
- buffer.dart: 0 / 260 → 260 / 261 (one while-loop-body line
  Dart coverage doesn't instrument distinctly; the loop's effect
  is exercised end-to-end by the reflow-pad test).
- Total project: 39.12% → 43.20%.
- pubspec.yaml `coverage_floor:` bumped 39 → 43.

Notes:
- The fake TerminalState (`_State`) is a per-file impl rather than
  a shared fixture; it stays close to the test that exercises it
  and avoids forcing other terminal tests to depend on a one-shape-
  fits-all stub.
- Tests that walk through `lineFeed` use `lineFeedMode: true` so
  the column resets between newlines — otherwise the saturated
  cursor X from a previous full-width write spills the next write
  onto an extra line via `writeChar`'s autoWrap branch.

This closes the `core/buffer/` sub-area for T-91 — every leaf file
in `lib/src/terminal/src/core/buffer/` is now at >= 96% line
coverage; the only outliers are Dart-coverage-instrumentation
quirks, not real gaps.

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: 39
+coverage_floor: 43
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads