fix(lint): clamp negative offsets in ColumnOfOffset

The binary-search rewrite only clamped the upper bound (offset >
len(Source)); a negative offset now echoed back out unclamped
(ColumnOfOffset(-5) == -4) instead of the pre-refactor behavior of
always returning 1 for any negative offset. No shipped rule call site
can currently trigger this (all pass an already-guarded, non-negative
value), but the contract should still match LineOfOffset's and the
prior implementation's handling of out-of-domain input.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UCh2j2oeSw6UgNsmpjRonL

Author: claude

internal/lint/file.go

@@ -554,6 +554,9 @@ func (f *File) ColumnOfOffset(offset int) int {
 	if offset > len(f.Source) {
 		offset = len(f.Source)
 	}
+	if offset < 0 {
+		offset = 0
+	}
 	// Reuses LineOfOffset's cached newline index via the same binary
 	// search instead of scanning backward from offset byte by byte —
 	// O(log n) in the newline count instead of O(line length). A single

internal/lint/lint_coverage_test.go

@@ -280,6 +280,14 @@ func TestColumnOfOffset_PastEOFClamps(t *testing.T) {
 	assert.Equal(t, 4, f.ColumnOfOffset(999))
 }
 
+func TestColumnOfOffset_NegativeOffsetClamps(t *testing.T) {
+	// A negative offset clamps to the start of the file (column 1),
+	// mirroring the upper-bound EOF clamp above.
+	f := &File{Source: []byte("line1\nline2\n")}
+	assert.Equal(t, 1, f.ColumnOfOffset(-1))
+	assert.Equal(t, 1, f.ColumnOfOffset(-100))
+}
+
 func TestColumnOfOffset_AtNewline(t *testing.T) {
 	// The newline itself sits at the end of its line.
 	f := &File{Source: []byte("ab\nc")}