fix: annotate only the changed lines

[dsherret]

The annotation is shown beside the file on the pull request, so the surrounding lines are already visible.

• The annotation now points at the first run of changed lines rather than the whole first hunk, so GitHub draws it next to the actual problem instead of after the trailing context lines.
• The message lists only the changed lines, formatted as context-free hunks like diff -U0, with a blank line after the summary.
• The line-endings-only detection reuses the same parsed changes instead of walking the diff a second time.

For example, on dprint/dprint#1247 the annotation moves from lines 8-14 to line 11 and becomes:

File is not formatted. Run `dprint fmt` to fix.

@@ -11 +11 @@
-##         GitHub Actions
+## GitHub Actions