grammar: a #tag starts after whitespace and with a letter, on every surface

The shared fixture went red on the server (run 8468: 5 failed), because the three
tag rules disagreed:
- core and web: any non-tag character counts as a boundary, so `(#todo)`
  and `end.#tag` are tags, and so is the `/#section` of a pasted URL;
- server: only whitespace counts, but `#1st` and `#_x` are tags.

A note's labels could therefore change every time it synced.

All three now share the strict rule: start of line or whitespace, then a
letter, then letters, digits, `_` and `-`. Nothing becomes a tag that wasn't
already one everywhere, and URL anchors stop becoming labels on desktop and
Android. The server's existing `http://x/#nope` test already expected this.

derive.rs's boundary, markdown.ts's lookbehind and tags.py's regex change
together; `_is_tag` goes because the regex now requires the letter. The
fixture flips `(#todo)`, `end.#tag` and its lift case, and adds the URL case.

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

frontend/src/notes/markdown.ts

@@ -46,15 +46,16 @@ export interface TaskMeta {
 // chance to start inside `**bold #x**`.
 //
 // The grammar MIRRORS `line_tags` in core/src/local/derive.rs, which is the definition:
-// a `#` at a word boundary (the preceding character is neither a tag character nor
-// another `#`, so `a#b` and `##x` are not tags), a letter immediately after it, then
+// a `#` at the start of the text or after whitespace (so `a#b`, `##x`, `(#x)` and the
+// `/#section` of a URL are not tags), a letter immediately after it, then
 // alphanumerics, `_` and `-`. Rust's `is_alphanumeric` is `Alphabetic | N`, hence the
-// property escapes rather than `\w` — and hence the `u` flag.
+// property escapes rather than `\w` — and hence the `u` flag. All three
+// implementations run core/testdata/grammar.json (notes/grammar.test.ts here).
 //
 // A heading cannot collide with this: `parseMarkdown` requires a space after the `#`s,
 // which `#tag` by definition does not have.
 const INLINE_RE =
-  /(`[^`]+`)|(\*\*[^*]+\*\*)|(\*[^*]+\*)|(_[^_]+_)|((?<![\p{Alphabetic}\p{N}_#-])#\p{Alphabetic}[\p{Alphabetic}\p{N}_-]*)/gu;
+  /(`[^`]+`)|(\*\*[^*]+\*\*)|(\*[^*]+\*)|(_[^_]+_)|((?<!\S)#\p{Alphabetic}[\p{Alphabetic}\p{N}_-]*)/gu;
 
 export function parseInline(text: string): InlineToken[] {
   const tokens: InlineToken[] = [];