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/command-palette.rst

@@ -0,0 +1,114 @@
+Command palette
+=================
+
+.. only:: man
+
+    Overview
+    --------------
+
+
+The command palette lets you browse, search and trigger all keyboard shortcuts
+and actions in |kitty| from a single searchable overlay. Press
+:sc:`command_palette` to open it (default: :kbd:`Ctrl+Shift+F3`).
+
+.. figure:: ../screenshots/command-palette.png
+    :alt: A screenshot of the command palette kitten
+    :align: center
+    :width: 100%
+
+    The command palette showing search results for "window close"
+
+All mapped actions (those with a keyboard shortcut) and unmapped actions (those
+available but not bound to any key) are listed, organized by category. Mouse
+bindings are shown in a separate section. Simply type to search, select a
+result, and press :kbd:`Enter` to run it.
+
+
+Searching
+-----------
+
+As you type into the search bar, the palette filters results in real time using
+word-level matching across three columns: key, action, and category.
+
+Multiple search terms are supported. Typing ``scroll page`` matches items that
+contain both "scroll" and "page" in any column. Items matching more of your
+search terms rank higher than those matching fewer.
+
+The search also handles compound tokens that contain delimiters such as
+underscores or slashes. For example, typing ``mouse_selection`` matches the
+full compound name as a unit. Typo tolerance is built in for words of four
+characters or longer.
+
+Matched characters are highlighted in the results so you can see exactly where
+each term matched.
+
+
+Keyboard controls
+-------------------
+
+The following keys are available while the command palette is open:
+
+.. list-table::
+    :widths: auto
+    :header-rows: 1
+
+    * - Key
+      - Action
+    * - Any text
+      - Filter results by typing a search query
+    * - :kbd:`Enter`
+      - Run the selected action
+    * - :kbd:`Escape`
+      - Clear the search query, or close the palette if the query is already empty
+    * - :kbd:`Up` / :kbd:`Ctrl+K` / :kbd:`Ctrl+P`
+      - Move selection up
+    * - :kbd:`Down` / :kbd:`Ctrl+J` / :kbd:`Ctrl+N`
+      - Move selection down
+    * - :kbd:`Page Up`
+      - Move selection up by a page
+    * - :kbd:`Page Down`
+      - Move selection down by a page
+    * - :kbd:`Home`
+      - Jump to the first result
+    * - :kbd:`End`
+      - Jump to the last result
+    * - :kbd:`Backspace`
+      - Delete the last character from the query
+    * - :kbd:`F12`
+      - Toggle display of unmapped actions
+    * - Mouse click
+      - Select and run the clicked action
+
+
+Unmapped actions
+------------------
+
+By default, the palette shows both mapped actions (those bound to a shortcut)
+and unmapped actions (those with no shortcut assigned). Unmapped actions appear
+with an ``(unmapped)`` label in the key column. Press :kbd:`F12` to toggle
+their visibility. This preference is remembered across sessions.
+
+Unmapped actions are useful for discovering functionality that you may not have
+configured a shortcut for. You can run them directly from the palette, or note
+the action name and add a mapping in :file:`kitty.conf`.
+
+
+Custom keyboard modes
+-----------------------
+
+If you have defined custom :ref:`keyboard modes <modal_mappings>` in your
+configuration, their bindings appear under separate mode headers in the palette.
+The ``push_keyboard_mode`` bindings are grouped with the target mode they
+activate, making it easy to see how to enter each mode alongside its shortcuts.
+
+
+Configuration
+--------------
+
+The default mapping to open the command palette is::
+
+    map kitty_mod+f3 command_palette
+
+You can change this in :file:`kitty.conf` like any other mapping. For example::
+
+    map ctrl+p command_palette

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.