dovenest

Search: cut the scaffolding around it

An over-engineering pass over the search feature. Same behaviour, ~580
fewer lines.

Server:
- one walk serves both scopes instead of two, so `both` reads the tree
  once; its file and match events now interleave
- axum's `Sse` does the framing and the headers; the hand-built
  `data: <json>` writer and the `SearchUser` extractor are gone
- `?roots=` (a comma list the client never sent more than one of)
  becomes `?root=`, and the counters lose their redundant `Arc`
- `SearchEvent::File` carries the sniffed `FileKind`, so the client
  needs no extension table of its own

Client:
- `EventSource` replaces ~80 lines of fetch + stream reader + manual
  SSE framing. It closes on the `error` that ends a stream, so the
  browser cannot reconnect and re-run the search. The cost is that a
  rejected request reports one generic message: an `EventSource`
  cannot read the server's JSON error body.
- `UrlSearchParams` replaces the hand-rolled hash-query parser
- the chunk-size feedback controller becomes one constant, the `Frag`
  enum folds into `highlight_html`, and its char-remap for characters
  whose lowercase expands is gone
- one staleness flag instead of two, one `Status::Done` instead of
  three variants, query words hoisted out of the per-row render

Dropped the committed design mockup. Added an integration test for the
merged walk, name-vs-path matching, the SSE framing and the rejection
paths.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>