Problem:
After clearing cursors, "]C" "[C" do nothing, although the previous
cursor positions ("gQ") may still be useful as marks.

Solution:
If multicursor is not active, "]C" "[C" iterate the
"nvim.multicursor.last" extmarks. They also work as an operator motion
("d]C"), since there is no cascade.

AI-assisted

Problem:
`nvim_mcursor` is not fully baked, and not strongly motivated,
currently. The main programmatic interface for cursors is the
"nvim.multicursor" extmarks namespace.

Solution:
Drop `nvim_mcursor`. Add an internal hook so that multicursor can react
when extmarks are added to the "nvim.multicursor" namespace.

TODO:
If extmarks/namespaces are clumsy to work with, we should _fix that_
rather than adding wrappers.

Problem:
- `[count]Q` and `{Visual}Q` are implemented as ad hoc utils, because
  `Q` is not an "operator".
- `Q` always "toggles", it is not "idempotent".
- Various *cursor-relative* marks (`'.`, `'[`) are not saved/restored
  per-cursor, so they don't work during cascade.
- Insert cascade clobbers the primary's `'^` and changelist.

Solution:
- Introduce the `zq{motion}` operator, which places cursors at every
  reachable `{motion}`.
  - `{Visual}Q` is now an alias to `{Visual}zqj`.
- Introduce `1Q` and `2Q` for idempotent cursor placement.
  - Drop `[count]Q`, it's redundant with `zqgn`.
- Classify cursor-relative marks as `type=motion` instead of
  `type=jump`. Save/restore per-cursor during cascade.

Enable EmmyLua's need-check-nil diagnostic and resolve nullable values
through more precise types, explicit invariants, and guards for missing
runtime state. Keep analyzer-specific workarounds local and update the
affected API documentation.

Exclude the bundled re.lua from analysis so its nil-check warnings do
not fail CI.

AI-assisted

Problem:
Unlike "gv", "gQ" does not restore the primary-cursor position. This is
inconvenient for workflows where you temporarily clear the cursors,
navigate somewhere, then want to return to the multicursor session.

Solution:
Store the primary-cursor position at extmark-id 1 of the
`'nvim.multicursor.last'` namespace.

Note: users that *don't* want to restore the primary-cursor position can
delete extmark-id 1 from the `'nvim.multicursor.last'` namespace just
before `gQ`. (Intentionally undocumented because this interface is not
final, and should not block Nvim 0.13 release.)

    vim.keymap.set('n', 'gQ', function()
      local ns = vim.api.nvim_create_namespace('nvim.multicursor.last')
      pcall(vim.api.nvim_buf_del_extmark, 0, ns, 1)
      return 'gQ'
    end, { expr = true })

Problem:
gQ is nonsense during an active multicursor session.

Solution:
Make it a no-op.

Problem:
Jumping to a cursor adds one at the old position, so navigation has a
side-effect. "]CQ" no longer deletes a cursor, and returning with "[C"
after wandering off adds a cursor wherever you were.

This is useful in some cases, but as a default it is surprising. The
default behavior should be simple and composable.

Solution:
Revert the "]C"/"[C" cursor-plopping behavior.
Document a mapping for the "rotate" behavior.

Problem:
If there are >85 cursors on-screen, the terminal barfs the termcodes to
the screen.

Solution:
Emit the coords in chunks. Only "0;4" clears, so the groups accumulate.

Problem:
Extra cursors use shape 29 (follow primary), so they render identically
to the primary instead of using hl-MCursor colors.

Solution:
Send MCursor fg/bg as sRGB via shapes 30/40 before placing cursors.
Terminals without 30/40 support ignore the color sequences.

AI-assisted