Parse per-file Word Count Target header and show progress

Header parsing now recognises a `Word Count Target:` line at any heading
level (## included) in addition to `# Title:`/`# Slug:`, and accepts one
or more leading `#`. The value parses a single number or a range —
"1500 - 2000", "1500-2000", "1,500 to 2,000" — into a min/max WordGoal
(stripped from the exported document like the other header lines).

When a target is set, the status bar shows a progress bar of the file's
prose word count (the body below the draft marker) against it: amber
under the target, green in range, blue over. Aliases "Word Count Goal:"
and "Word Count:" also work.

Adds preprocess tests for the range forms, ## heading level, alias, and
absence; 44 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N9kRuP7JvXoUGdNNeg5ZSs

README.md

@@ -110,6 +110,22 @@ The status bar shows a live **word count** for the current file, plus the net
 words added (or removed) to it since the current session started. The count
 updates as you type, including unsaved edits.
 
+### Per-file word-count targets
+
+Give a file a target by adding a **`Word Count Target:`** line to its header (at
+any heading level):
+
+```markdown
+## Word Count Target: 1500 - 2000
+```
+
+A **progress bar** then appears in the status bar, comparing the file's **prose**
+word count (the body below the draft marker, so header metadata isn't counted)
+against the target: **amber** while under the target, **green** once inside the
+range, **blue** when over it. A single number (`## Word Count Target: 1800`) sets
+a point goal; ranges accept `-`, `–`, `to`, and grouped digits (`1,500`). Like
+the other header lines, the target is stripped from the exported document.
+
 ## Spelling (offline)
 
 Spelling is checked **live as you type**, entirely offline — no server, no