test sweep: cover core/input/ keytab + handlers (T-91)

Adds test/terminal/input/input_test.dart — 58 unit tests covering
the keytab tokenizer, parser, unescape helper, KeytabRecord
toString shapes, Keytab.find modifier-matching rules, and the four
TerminalInputHandler implementations (CascadeInputHandler,
KeytabInputHandler, CtrlInputHandler, AltInputHandler).

Highlights:

- keytabUnescape: every documented backslash escape + \xHH hex.
- LineReader: peek/take/done, whitespace skip, readString
  (alphanumeric/underscore), readUntil (both exclusive and
  inclusive variants).
- tokenize: keyboard-name and key-define lines, comment + blank
  stripping, shortcut vs string actions, error paths on malformed
  input.
- KeytabParser: full mode-flag matrix, error paths on every
  defensive throw reachable through the public addTokens API
  (stray non-keyboard token, missing colon, modeStatus value other
  than '+'/'-', non-mode token after modeStatus, action token of
  wrong type, second token of wrong kind for both _parseName and
  _parseKeyDefine).
- KeytabRecord.toString covers every supported flag (Alt, Control,
  Shift, AnyMod, Ansi, AppScreen, KeyPad, AppCuKeys, AppKeyPad,
  NewLine, Mac).
- Keytab.find: -Shift / +AnyMod / -AnyMod gating, mode-flag
  filters (newLine, appKeyPad, appScreen, macos, appCursorKeys,
  keyPad), -Ansi (VT52) skip, fallthrough to fallback record,
  null when no key matches.
- KeytabInputHandler: every modifier combination's `*` placeholder
  expansion (1..8 inclusive), default-keytab fallback, no-match
  null, no-* passthrough.
- CtrlInputHandler: A..Z → 0x01..0x1A; null without ctrl, with
  shift / alt, or on non-letter keys.
- AltInputHandler: A..Z → ESC + uppercase; null without alt, with
  shift / ctrl, on macOS, or on non-letter keys.
- defaultInputHandler integration: keytab routing, fallthrough to
  CtrlInputHandler.

Coverage delta:
- core/input/handler.dart: 4/54 → 54/54.
- keytab.dart: 0/29 → 29/29.
- keytab_record.dart: 0/44 → 44/44.
- keytab_token.dart: 0/82 → 80/82 (the two remaining lines are
  defensive throws inside `_parseKeyboardNameDefine` /
  `_parseKeyDefine` that are unreachable from tokenize() — the
  callers only enter those functions after the `_isKeyboardNameDefine`
  / `_isKeyDefine` guards in the same file, so the inner readString
  always matches).
- keytab_parse.dart: 0/65 → 63/65 (the two remaining lines mirror
  the same shape — _parseName and _parseKeyDefine both check the
  first token's type, but addTokens only delegates to them after
  matching that type, so the throws are dead defensive code).
- keytab_default.dart: 0/4 unchanged — that's the file's own
  `void main()` debug entrypoint that prints the parsed default
  keytab; not part of the runtime contract.
- keytab_escape.dart: 0/14 → 14/14.
- Total project: 49.31% → 52.53%; coverage_floor bumped 49 → 52.

The 4 dead defensive throws are flagged but not removed in this
commit — they're a code-style call (defensive paranoia vs. dead-
code cleanup) that belongs in a separate review, not folded into a
test sweep.

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: 49
+coverage_floor: 52
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads