Problem:
- Dot-repeat of a Visual selection prepared by a `<Cmd>` mapping,
results in E1255 and leaves Visual mode active.
- Visual-mode capture is implemented as a second, parallel "capture
engine": it re-composes a CmdSpec from cmdarg_T per key and decides
replayability from its own table of "void" key classes.
Solution:
- Model nested `normal_execute()` as a `CmdFrame` stack,
instead of a single module-scoped `stage`.
- Capture `<Cmd>` ":norm …" commands as subatoms from its nested frames.
- Produce every command exactly once; atom_push_raw() routes the atom to
the Visual composite while a selection is open, like it already does
for mapping composites.
- `v/pat<CR>d` is now repeatable and cascades.
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.
