test sweep: cover core/(root) — cell, cursor, charset, tabs, reflow (T-91)

Adds test/terminal/core/core_test.dart — 36 unit tests across the
small standalone files that sit directly under
`lib/src/terminal/src/core/*.dart`:

- CellData (constructor + empty + getHash + toString),
- CursorStyle (default ctor, every set/unset attr getter pair, all
  three colour-mode setters per channel, reset, the .empty
  singleton) + CursorPosition,
- Charset (translate, designate/use, save/restore, asciiTranslator,
  decSpecGraphicsTranslator with in-table, out-of-table, and high-
  codepoint paths),
- TabStops (default 8-column grid, find with empty-range / out-of-
  bounds / no-stop-in-range cases, setAt/clearAt/clearAll/reset),
- reflow (empty input, single-line passthrough, grow, shrink-with-
  split, wrapped-run continuation, wide-char boundary on the new
  width, inner wide-char clamp during _addPart, anchor reparent on
  the main path, anchor reparent past trimmedLength).

Two source-side cleanups folded in:
- `CursorStyle.isItalis` was a defined-but-never-called getter
  with a typo. No external callers reference it; renamed to
  `isItalic` in the same change as the test that exercises it.
- `_LineBuilder.isEmpty` in reflow.dart was dead — the only callers
  use `isNotEmpty` or check `_lines.isNotEmpty` separately.
  Removed.

Coverage delta:
- cell.dart: 3/7 → 7/7.
- charset.dart: 12/25 → 25/25.
- cursor.dart: 2/62 → 62/62.
- tabs.dart: 0/23 → 23/23.
- reflow.dart: 24/72 → 71/71 (file shrank by one line after the
  isEmpty getter removal).
- Total project: 52.72% → 54.62%; coverage_floor bumped 52 → 54.

Note for follow-up (not blocking): the post-loop "anchor.x >= to"
branch in reflow's `_addPart` reparents anchors past trimmedLength
onto whatever builder line is active at that moment. If no
subsequent content is added (no wrapped continuations after the
last shrink iteration), that builder line is never emitted by
`finish()` and the anchor lands on a dangling reference. The path
is exercised by the new test, but the contract it implements is
arguably broken — anchors that should follow the source content
end up off the visible buffer. Worth a separate ticket if real
terminals trip it.

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: 52
+coverage_floor: 54
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads