fix(multicursor): visual "gv", change-marks '[,'] #41988

Problem:
- `gv` reselects only primary's region, it doesn't have per-cursor
  behavior. '< '> are buffer-global and the sandbox restores the
  primary's marks. The Visual atom containing `gv` was voided anyway, so
  it never cascaded.
- Same for change-marks (`'[ '] '.`): they are stale after edits from
  other cursors, not managed per-cursor.
- `McSandbox` saves "editor state", which should be owned by the
  `Context` module.

Solution:
Lift more state from `McSandbox` into `Context`. `kCtxCursor` (position,
curswant) and `kCtxMarks` (Visual area, change-marks), are owned by
`Context` now, as an "in-memory" special-case (TODO: support it for the
"serialized" case?).

- Classify a `gv` selection as replayable. Manage its state per-cursor.
  Dot-repeat "." after `gv` behaves like Vim (the "1v" fallback).
- Save/restore change-marks per-cursor. The primary's change-marks are
  shifted by edits from other cursors during the cascade.

`McSandbox` still contains (for now) the view and replay isolation
(typeahead/mode, redobuff, search patterns, the active selection).

Bonus:
- `visualmode()` is per cursor.
- A cursor's `curswant` follows its column when other cursors' edits
  shift it, instead of the column captured after its own replay.

0 files changed, 0 insertions, 0 deletions