palette: cut the per-keystroke cost of project-wide symbol search

Reported as sluggish up/down navigation in project scope. Measuring it turned up
two things:

* The arrow keys themselves aren't the problem, and it isn't list length: a
  synthetic down-key benchmark reads a flat ~62 ms/key from 20 to 400 options in
  BOTH scopes. That is a fixed floor in pilot.press() (Textual awaits a full
  refresh cycle per synthetic press), so the harness can't isolate real key
  latency — the number says nothing about the app.
* What IS measurably worse in project scope is the work done per typed
  character: 29.5 ms vs 17.8 ms locally.

Fix the part that's real. The trigram index already guarantees every hit contains
the query, so ranking only has to ORDER them — an exact-substring rank (match
position, then name length) costs one find() per row instead of a full fuzzy pass,
and fetching 3x the display limit instead of 10x cuts the candidate set. As a
bonus the highlight spans come straight from the match position.

Per-keystroke _apply over a 30k-entry, 6-binary index, typing "m"->"main_":
  before   project  ~29.5 ms
  after    project  10.8 / 11.2 / 11.9 / 13.2 / 14.2 ms
  (local, unchanged, for reference: 13.8 / 14.3 / 15.2 / 16.2 / 38.8 ms)

Project scope is now cheaper per keystroke than local scope, whose fuzzy pass
over every function is the remaining hot spot. Suite 191/0.

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