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.

runtime/lua/vim/_meta/api_keysets.gen.lua

@@ -105,6 +105,7 @@ error('Cannot require a meta file')
 --- |'ChanClose'
 --- |'ChanInfo'
 --- |'ChanOpen'
+--- |'CmdAtom'
 --- |'CmdUndefined'
 --- |'CmdlineChanged'
 --- |'CmdlineEnter'

runtime/lua/vim/_meta/events.lua

@@ -4,6 +4,23 @@
 -- See also `vim.api.keyset.events` in `api_keysets.gen.lua`.
 error('Cannot require a meta file')
 
+--- Data for the CmdAtom event.
+--- @class vim.event.cmdatom.data
+--- @field arg? string Typed operand of `cmd` ("fx" => "x").
+--- @field atoms? vim.event.cmdatom.data[] Subatoms of a composite (mapping, Visual sequence).
+--- @field cascade boolean Queued for multicursor replay.
+--- @field changed boolean Changed the buffer.
+--- @field cmd? string Command/motion/object name ("w", "f", "iw", "gJ").
+--- @field count? integer Effective count.
+--- @field keys string Resolved keysequence, raw bytes: feed to nvim_feedkeys() to replay.
+--- @field lhs? string Mapping LHS or macro register ("gj", "@q"). Raw bytes, like `keys`.
+--- @field motionforce? 'v'|'V'|'<C-V>' forced-motion type.
+--- @field operator? string Operator name ("d", "g~", "g@"). key-notation.
+--- @field pending? 'operator'|'visual' Mapping ended mid-operation.
+--- @field reg? string Register name.
+--- @field text? string Inserted text, or the Ex/search cmdline.
+--- @field type 'command'|'ex'|'insert'|'jump'|'mapping'|'motion'|'mouse'|'operator'|'scroll'|'visual'
+
 --- @class vim.event.lspattach.data
 --- @field client_id integer
 

runtime/lua/vim/_meta/options.gen.lua

@@ -2193,6 +2193,7 @@ vim.go.ei = vim.go.eventignore
 --- 	`ChanClose`,
 --- 	`ChanInfo`,
 --- 	`ChanOpen`,
+--- 	`CmdAtom`,
 --- 	`CmdUndefined`,
 --- 	`CmdlineChanged`,
 --- 	`CmdlineEnter`,