fix: address docs checker review findings

Check every link reference definition, decode destinations per CommonMark,
report correct lines for empty headings, autolinks and nested definitions,
reject symlinked record directories and wrong-case path components, and
refuse titles that cannot pass. Only top-level headings count (spec section 6).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/docs/links_test.go

@@ -16,6 +16,12 @@ func TestLinkDestinations(t *testing.T) {
 		{"#anchor", ""},
 		{"mailto:a@example.com", ""},
 		{"https://example.com/a?b=1#c", ""},
+		{`/docs/spec\.txt`, ""},
+		{"/docs/spec.txt", ""},
+		{"/docs/spec.txt", ""},
+		{`/docs\\spec.txt`, "backslash"},
+		{"/docs/SPEC.txt", "does not exist"},
+		{"/Docs/spec.txt", "does not exist"},
 		{"foo.md", "relative"},
 		{"./foo.md", "relative"},
 		{"../docs/spec.txt", "relative"},
@@ -64,6 +70,11 @@ func TestLinkForms(t *testing.T) {
 		{"image", "![x](foo.md)", []string{":35: relative"}},
 		{"autolink", "<ftp://example.com/x>", []string{":35: scheme"}},
 		{"reference link and definition", "[x][r]\n\n[r]: foo.md", []string{":35: relative", ":37: relative"}},
+		{"duplicate definition is checked", "[a]: /docs/spec.txt\n\n[a]: ../bad.md", []string{":37: relative"}},
+		{"definition in a blockquote", "> [Zed]: ../bad.md", []string{":35: relative"}},
+		{"definition-like line in code", "```\n[a]: x\n```\n\n[a]: ../bad.md", []string{":39: relative"}},
+		{"autolink on a later line", "Para one\nline two\n<ftp://example.com/x>", []string{":37: scheme"}},
+		{"image without alt on a later line", "Para one\nline two\n![](foo.md)", []string{":37: relative"}},
 		{"unused definition", "[r]: foo.md", []string{":35: relative"}},
 		{"link in a list", "* [x](foo.md)", []string{":35: relative"}},
 		{"bare URL and raw HTML are not checked", "http://u:p@example.com and @user #123 abc1234\n\n<a href=\"foo.md\">x</a>", nil},