feat(input)!: CmdAtom event #41297

Problem:
There is no unified notion of a "user action".

Vim processes input by one-char-at-a-time, and mostly throws away any
hints it might gather about the user's action, with one exception: it
stores the last _edit_ action (the "redo buffer", encoded as
unstructured `["x][v][count]body` bytes).

Plugins can only observe individual keys (vim.on_key) and high-level
effects (TextChanged, CursorMoved).

Solution:
- Users can subscribe to `CmdAtom` events to handle any user action.
  - Event is deferred; handlers cannot cancel or interfere with user
    actions.
- Capture `CmdSpec` from the normal/insert/visual subsystems.
  - typeahead/readahead stay unstructured (`buffheader_T`): they are key
    streams, not commands.
  - the redo/record buffers become `StringBuilder`: fewer
    allocations/copies.
- Repurpose the input/redo engine to accept `CmdSpec` objects.

"atom": one repeatable unit of user input, as a resolved (post-mapping)
keysequence plus structured fields. Only user actions, not `:normal`,
API calls, or non-"t" `feedkeys`.

BREAKING: dot-repeat of an Insert session, replays the entire session
including cursor-moves (:help ins-repeat).

BREAKING: dot-repeat of a Visual operation, replays the selection
instead of operating on a fixed-size region.

test/functional/lua/comment_spec.lua

@@ -426,14 +426,14 @@ describe('commenting', function()
       eq(get_lines(), { '# aa', '#  aa', '#   aa', '', '  aa', ' aa', 'aa' })
       eq(get_cursor(), { 1, 0 })
 
-      -- Dot-repeat after first application in Visual mode should apply to the same
-      -- relative region
+      -- Dot-repeat after first application in Visual mode applies to the paragraph at cursor (not
+      -- a fixed-size region).
       feed('.')
       eq(get_lines(), example_lines)
 
       set_cursor(3, 0)
       feed('.')
-      eq(get_lines(), { 'aa', ' aa', '  # aa', '  #', '  # aa', ' aa', 'aa' })
+      eq(get_lines(), { '# aa', '#  aa', '#   aa', '', '  aa', ' aa', 'aa' })
     end)
 
     it("respects 'commentstring'", function()