Enable EmmyLua's assign-type-mismatch diagnostic after correcting the
runtime annotations and assignments it flags.

Correct option, tuple, and return types, and use separate locals when a
value changes representation. Add guards and narrow casts where needed
so the runtime passes the stricter check. Regenerate the affected docs.

AI-assisted

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:
Cmdline area shows stale ":" after backspacing out of the command line.

Solution:
Clear the command line for empty commands. Note that `:<CR>` will now
clear the command line too.

Ref #6645

Problem:
When a window is resized it takes space from the window right/below first,
and only falls back to the window left/above when there is no more room.
Sometimes a user wants the space to come from a specific direction.

Solution:
Add nvim_win_resize(win, width, height, {anchor}) which resizes a window
with a choosable anchor edge, letting a window grow leftwards or upwards
by taking space from the window to the left or above first. The default
anchor reproduces nvim_win_set_width()/nvim_win_set_height().

Problem:  Message before empty input() is not visible.
Solution: Route to dialog window with active prompt (hl_id >= 0).

Problem:  - "bufwrite" message identifier is encoded in the message ID
            of a "progress" kind message (since ff68fd6b); UI2 does not
            allow routing by message ID.
          - No documented way to set a default message target for all
            but a few kinds (without copying all of |ui-messages| kinds
            to cfg.msg.targets).
          - A user adding a message route for the documented empty ""
            kind can result in unexpected behavior.
          - Showing duplicate message (x) indicator in msg and cmd
            targets simultaneously is unsupported.
          - Manually triggering CursorMoved autocommand to add matchparen
            highlighting in the cmdline.
Solution: - Match cfg.msg.targets keys as Lua pattern to a message ID.
          - Recognize "default" as key in cfg.msg.targets, drop the
            undocumented cfg.msg.target field.
          - Don't try to get configured target for "" message kind/trigger.
          - Maintain msg indicator virtual text for the cmd and msg target.
          - Add matchparen highlighting by directly calling the Lua module
            (possible since b813c7e0).

Problem:  'showcmd' causes flickering when pressing "g<" to enter the
          pager when the cmdline is expanded for messages.
          Initial keypress for an incomplete mapping is not giving 'showcmd'
          feedback while cmdline is expanded for messages (which is only
          dismissed upon the vim.on_key callback after 'timeoutlen').

Solution: Delay dismissing expanded cmdline when vim.on_key() callback
          receives "g".
          Place 'showcmd' "last" virtual text during expanded cmdline.

Problem: Cmdline is not redrawn after an empty message clears it.
Remembered last drawn cursor position may be outdated but
equal to the current cmdline content with UI2.
Solution: Ensure cmdline is redrawn after an empty message clears it.
Compare wanted cursor position with actual cursor position.

Problem:  - With expanded messages exceeding cfg.msg.cmd.height, entering
            the cmdline scrolls to the bottom and expands to the full "cmd"
            buffer text height.

          - Cursor in the pager is not always at the last message and at
            the bottom of the window when appending to the pager.

          - unreliable test: messages2_spec: "closed msg window timer removes
            empty lines".
Solution: - Achieve separation of the cmdline and message text by moving
            messages to the dialog window when entering the cmdline below
            expanded messages.

          - Set cursor to start of the first message only when first
            entering the pager. Use `norm! zb` to position last message
            at the bottom of the window (which shouldn't crash anymore
            since 911337eb).

          - Increase cfg.msg.msg.timeout used in the test file.