fix(heading-title-case): skip alphanumeric identifiers (e.g. 4a, v2) for title-case

Words like "4a" were incorrectly flagged as needing capitalization. Skip
segments that contain both a digit and a letter (alphanumeric identifiers).
Add unit tests, positive fixture heading, and functional fix test.

Author: cypher0n3

Makefile

@@ -131,7 +131,7 @@ test-rules:
 	@node --test test/markdownlint-rules/*.test.js
 
 # Unit test coverage for markdownlint-rules/*.js (fails if any file < 90% lines/statements).
-# Requires: Node.js, npm; run 'npm install' first.
+# Requires: Node.js, npm; run 'npm install' first. Use Node 20 or 22 for coverage (c8 fails on Node 25).
 test-rules-coverage:
 	@command -v node >/dev/null 2>&1 || { \
 		echo "Error: node not found. Install Node.js and run npm install."; \

markdownlint-rules/heading-title-case.js

@@ -172,11 +172,17 @@ function isPhaseLabelSegment(j, core, previousWordCore) {
   return j === 0 && core.length === 1 && /^[A-Za-z]$/.test(core) && LABEL_PARENT_WORDS.has(previousWordCore);
 }
 
-/** True if this segment should be skipped for title-case (entity, empty, non-alpha, or scientific notation). */
+/** Alphanumeric identifier (e.g. 4a, v2, 2b); skip so "4a" is not flagged for capitalization. */
+function isAlphanumericIdentifier(core) {
+  return /[0-9]/.test(core) && /[a-zA-Z]/.test(core);
+}
+
+/** True if this segment should be skipped for title-case (entity, empty, non-alpha, scientific notation, or alphanumeric identifier). */
 function shouldSkipSegmentForTitleCase(rawSeg, core) {
   return isHtmlEntity(rawSeg)
     || !core || !/[a-zA-Z]/.test(core)
-    || RE_SCIENTIFIC.test(core);
+    || RE_SCIENTIFIC.test(core)
+    || isAlphanumericIdentifier(core);
 }
 
 /**

md_test_files/positive_general.md

@@ -154,6 +154,10 @@ Should not catch.
 
 Scientific notation (e.g. 5e, 1e6) in headings is not title-cased; the "e" is left as-is.
 
+### 9. Section 4a and Results
+
+Alphanumeric identifiers (e.g. 4a, v2, 2b) in headings are not title-cased; they are left as-is.
+
 ## Zero-Indexed Heading Numbering
 
 When the first numbered heading in a section starts at 0, the rule treats that section as 0-based and does not report errors.