refactor: two small pass-2 review cleanups

Pass 2 of code review on PR #732 found two more instances of patterns
pass 1 had just fixed elsewhere:

- internal/rules/tokenbudget/rule.go: Check() still inlined its own
  copy of the TokensPerWord-fallback resolution that
  effectiveTokensPerWord() (pass 1) was extracted specifically to
  de-duplicate from tokenCount/modeLabel/definitelyUnderBudget — a
  4th, un-consolidated copy. Now calls the shared helper too.

- pkg/markdown/flavor/detect.go: restoring lineStartOf's
  negative-offset clamp (pass 1) copied the same clamp logic LineCol
  already had a few lines below instead of sharing it. Extracted
  clampOffset, used by both.

Author: claude

internal/rules/tokenbudget/rule.go

@@ -81,11 +81,7 @@ func (r *Rule) Check(f *lint.File) []lint.Diagnostic {
 	modeLabel := r.modeLabel()
 
 	overage := count - budget
-	tpw := r.TokensPerWord
-	if tpw <= 0 {
-		tpw = defaultTokensPerWord
-	}
-	wordsOver := int(math.Ceil(float64(overage) / tpw))
+	wordsOver := int(math.Ceil(float64(overage) / r.effectiveTokensPerWord()))
 	if wordsOver < 1 {
 		wordsOver = 1
 	}
@@ -149,8 +145,8 @@ func (r *Rule) definitelyUnderBudget(source []byte, budget int) bool {
 }
 
 // effectiveTokensPerWord returns r.TokensPerWord, falling back to
-// defaultTokensPerWord when unset — the same resolution definitelyUnderBudget,
-// tokenCount, and modeLabel all need for heuristic mode.
+// defaultTokensPerWord when unset — the same resolution Check,
+// definitelyUnderBudget, tokenCount, and modeLabel all need.
 func (r *Rule) effectiveTokensPerWord() float64 {
 	tpw := r.TokensPerWord
 	if tpw <= 0 {

pkg/markdown/flavor/detect.go

@@ -338,13 +338,21 @@ func nodeByteRange(n ast.Node) (int, int) {
 }
 
 func lineStartOf(source []byte, offset int) int {
+	offset = clampOffset(source, offset)
+	return bytes.LastIndexByte(source[:offset], '\n') + 1
+}
+
+// clampOffset bounds offset to [0, len(source)] so a caller that
+// looks at byte -1 or one past EOF still gets a valid index to slice
+// or index with. Shared by lineStartOf and LineCol.
+func clampOffset(source []byte, offset int) int {
 	if offset < 0 {
-		offset = 0
+		return 0
 	}
 	if offset > len(source) {
-		offset = len(source)
+		return len(source)
 	}
-	return bytes.LastIndexByte(source[:offset], '\n') + 1
+	return offset
 }
 
 // firstTextStart returns the byte offset of the first descendant Text
@@ -388,12 +396,7 @@ func isASCIISpace(b byte) bool {
 // line numbers when producing line-level edits; also used internally
 // by every block / inline / makeFinding helper.
 func LineCol(source []byte, offset int) (line, col int) {
-	if offset < 0 {
-		offset = 0
-	}
-	if offset > len(source) {
-		offset = len(source)
-	}
+	offset = clampOffset(source, offset)
 	// bytes.Count and bytes.LastIndexByte are SIMD-accelerated on
 	// amd64; a manual byte-by-byte loop over the same prefix does not
 	// vectorize. See docs/development/high-performance-go.md