remove dead defensive throws in keytab tokenizer + parser

Four `throw` sites in `core/input/keytab/` were unreachable through
the public API:

- `keytab_token.dart` `_parseKeyboardNameDefine` and `_parseKeyDefine`
  each tested `reader.readString() == 'keyboard'` / `'key'` after
  the caller in the same file (`tokenize`) had already gated entry
  on `_isKeyboardNameDefine` / `_isKeyDefine`. Both checks
  redundantly re-derived a fact already established a function
  call earlier; the `else { throw }` was dead code.
- `keytab_parse.dart` `_parseName` and `_parseKeyDefine` checked
  the first token's type, but `addTokens` only delegates to those
  functions after `peek().type` matches the expected kind. Same
  pattern: the throw protects an invariant the caller already
  enforces.

Surfaced while bringing `core/input/` to ~100% coverage. Per the
"near-perfect discipline" / "no carve-outs" rules, dead defensive
code is cleaned, not skipped — the surrounding callers in the same
file are tight enough that introducing a real callsite gap would
be a localised and obvious bug, not a silent failure rescued by
these guards.

The two `else`-throw sites in keytab_token.dart fold into a single
unconditional `reader.readString()` (consume the leading word) +
`yield` of the matching token type. The two type-check throws in
keytab_parse.dart fold into an unconditional `reader.take()` to
skip the already-validated token.

All public-API ParseError paths exercised by `core/input/`'s
unit tests still throw correctly — they're guarded by the second
check in each function (the action-token type check after
modeStatus loops, and the input-token check in _parseName).

After cleanup:
- keytab_token.dart: 80 / 80
- keytab_parse.dart: 63 / 63

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

lib/src/terminal/src/core/input/keytab/keytab_parse.dart

@@ -55,9 +55,8 @@ class KeytabParser {
   }
 
   void _parseName(TokensReader reader) {
-    if (reader.take()!.type != KeytabTokenType.keyboard) {
-      throw ParseError();
-    }
+    // Caller (`addTokens`) has already gated on the keyboard token type.
+    reader.take();
 
     final name = reader.take()!;
     if (name.type != KeytabTokenType.input) {
@@ -68,9 +67,8 @@ class KeytabParser {
   }
 
   void _parseKeyDefine(TokensReader reader) {
-    if (reader.take()!.type != KeytabTokenType.keyDefine) {
-      throw ParseError();
-    }
+    // Caller has already gated on the keyDefine token type.
+    reader.take();
 
     final keyName = reader.take()!;
 

lib/src/terminal/src/core/input/keytab/keytab_token.dart

@@ -114,13 +114,11 @@ bool _isKeyDefine(String line) {
 }
 
 Iterable<KeytabToken> _parseKeyboardNameDefine(String line) sync* {
+  // Caller has already gated on `_isKeyboardNameDefine`, so the leading
+  // word is guaranteed to be 'keyboard'. Just consume it.
   final reader = LineReader(line.trim());
-
-  if (reader.readString() == 'keyboard') {
-    yield KeytabToken(KeytabTokenType.keyboard, 'keyboard');
-  } else {
-    throw TokenizeError();
-  }
+  reader.readString();
+  yield KeytabToken(KeytabTokenType.keyboard, 'keyboard');
 
   reader.skipWhitespace();
 
@@ -128,13 +126,10 @@ Iterable<KeytabToken> _parseKeyboardNameDefine(String line) sync* {
 }
 
 Iterable<KeytabToken> _parseKeyDefine(String line) sync* {
+  // Caller has already gated on `_isKeyDefine`.
   final reader = LineReader(line.trim());
-
-  if (reader.readString() == 'key') {
-    yield KeytabToken(KeytabTokenType.keyDefine, 'key');
-  } else {
-    throw TokenizeError();
-  }
+  reader.readString();
+  yield KeytabToken(KeytabTokenType.keyDefine, 'key');
 
   reader.skipWhitespace();