Command palette: word-level search with multi-token cross-column matching

Implement a word-level scoring engine for the command palette that replaces
the previous FZF-based approach. Query tokens are matched against pre-tokenized
words in each column (key, action, category) with exact, prefix, and
edit-distance scoring. Multiple search terms are supported with cross-column
matching — items matching more tokens rank higher. Compound query tokens
containing delimiters (e.g. mouse_selection) are matched as units.

Add comprehensive tests using a Go builder API instead of raw JSON blobs,
covering single-token, multi-token, partial-match, ranking, mouse binding,
and unmapped action scenarios.

Add documentation for the command palette kitten.

docs/kittens_intro.rst

@@ -13,6 +13,7 @@ Extend with kittens
    kittens/themes
    kittens/choose-fonts
    kittens/hints
+   kittens/command-palette
    kittens/quick-access-terminal
    kittens/choose-files
    kittens/panel
@@ -54,6 +55,11 @@ Some prominent kittens:
     filenames, words, lines, etc. from the terminal screen.
 
 
+:doc:`Command palette <kittens/command-palette>`
+    Browse, search and trigger all keyboard shortcuts and actions from a
+    single searchable overlay.
+
+
 :doc:`Quick access terminal <kittens/quick-access-terminal>`
     Get access to a quick access floating, semi-transparent kitty window
     with a single keypress.