test: improve mutation coverage across 4 modules

Mutation testing (56 mutations, 10 modules) identified 7 surviving
mutations. Added 5 targeted tests to kill all actionable survivors.
Remaining 2 are logically equivalent condition reorderings in
filter.go. Overall mutation score: 96.4%.

New tests:
- TestHasMalformedYAML: YAML unmarshal error path
- TestInvalidateInvalidCheatpath: cheatpath.Validate() delegation
- TestIndentTrimsWhitespace: TrimSpace behavior
- TestColorizeDefaultSyntax: default "bash" lexer
- TestColorizeExplicitSyntax: lexer differentiation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/sheet/parse_test.go

@@ -93,3 +93,20 @@ To foo the bar: baz`
 		t.Errorf("failed to parse text: want: %s, got: %s", markdown, text)
 	}
 }
+
+// TestHasMalformedYAML asserts that an error is returned when the frontmatter
+// contains invalid YAML that cannot be unmarshalled
+func TestHasMalformedYAML(t *testing.T) {
+
+	// stub cheatsheet content with syntactically invalid YAML between the
+	// delimiters (a bare tab character followed by unquoted colon)
+	markdown := "---\n\t:\t:\n---\nBody text here"
+
+	// parse the frontmatter
+	_, _, err := parse(markdown)
+
+	// assert that an error was returned due to YAML unmarshal failure
+	if err == nil {
+		t.Error("failed to error on malformed YAML frontmatter")
+	}
+}