feat: add per-mapping --allow-fallback for layout-independent shortcuts

Add --allow-fallback option to the map command that controls shifted
and ascii (alternate_key) fallback for individual key mappings.

For non-Latin keyboard layouts, when the current layout key is
non-ascii (codepoint > 127 and < 0xE000), the alternate_key from
the base layout is used for matching if the mapping opts in via
--allow-fallback=shifted,ascii.

Default kitty bindings use --allow-fallback=shifted,ascii so they
work out of the box with non-Latin layouts. User custom mappings
default to --allow-fallback=shifted (preserving existing shifted_key
behavior without ascii fallback).

--allow-fallback=none disables all fallback for a mapping.

Python side: parse_options_for_map() in options/utils.py handles flag
parsing, ShortcutMapping uses it in __init__. get_shortcut() filters
candidates by per-mapping allow_fallback.

Go side: ParseMap() handles --allow-fallback, KeyAction stores
AllowFallback, ShortcutTracker.Match passes it to matching.
MatchesParsedShortcut defaults to shifted,ascii for hardcoded shortcuts.

Migrated kittens (themes, command_palette, diff, choose_files) to
use ShortcutTracker with configurable map entries.

Tests added for Python (5 test methods) and Go (ParseMap + key matching).

kittens/command_palette/main.py

@@ -6,11 +6,38 @@ import sys
 from functools import partial
 from typing import Any
 
+from kitty.conf.types import Definition
 from kitty.fast_data_types import add_timer, get_boss
 from kitty.typing_compat import BossType
 
 from ..tui.handler import result_handler
 
+definition = Definition(
+    '!kittens.command_palette',
+)
+
+agr = definition.add_group
+egr = definition.end_group
+map = definition.add_map
+
+# shortcuts {{{
+agr('shortcuts', 'Keyboard shortcuts')
+
+map('Move selection up',
+    'selection_up --allow-fallback=shifted,ascii ctrl+k selection_up',
+    )
+map('Move selection up',
+    'selection_up --allow-fallback=shifted,ascii ctrl+p selection_up',
+    )
+map('Move selection down',
+    'selection_down --allow-fallback=shifted,ascii ctrl+j selection_down',
+    )
+map('Move selection down',
+    'selection_down --allow-fallback=shifted,ascii ctrl+n selection_down',
+    )
+
+egr()  # }}}
+
 
 def collect_keys_data(opts: Any) -> dict[str, Any]:
     """Collect all keybinding data from options into a JSON-serializable dict."""
@@ -188,3 +215,5 @@ elif __name__ == '__doc__':
     cd['options'] = OPTIONS
     cd['help_text'] = help_text
     cd['short_desc'] = help_text
+elif __name__ == '__conf__':
+    sys.options_definition = definition  # type: ignore