Problem: Argument type mismatches are excluded from EmmyLua checks.

Solution: Correct runtime annotations and calls, document local checker
workarounds, and enable the diagnostic.

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

Enable checks for unused code, deprecated calls, return types, and
annotations.

Fix the warnings and replace the deprecated highlight function. Keep
exceptions where old APIs are still needed or the checker gets it wrong.

AI-assisted

Problem:
Options parsing is still painful for dict-style options.

Solution:
schema-maxxing => better `opt:get()` (will be the basis for `vim.o()`),
unified (and more-detailed) err msgs.

- Drop bespoke structure-builder in `_core/options.lua`.
- Define `schema` for all non-primitive options (except 'guicursor' and
  statusline-style options); generate reified keysets `OptKeyDict`).
  - Generate 'fillchars' => `fcs_tab`, 'listchars' => `lcs_tab`.
- `nvim_set_option_value`:
  - Return the improved structures. Also from `vim.opt.x:get()`.
  - Eliminate api <=> lua roundtrip, centralize option structure
    handling.
- Improve/unify errors.
  - Bump ERR_BUFLEN 80 → 256 so the "one of" list isn't truncated.
- Eliminate old 'diffopt' order-dependence (`iwhiteall` before `iwhite`)

Error samples:

    Typed-key path (opt_strings_check → diffopt/mousescroll/breakindentopt):
    E474: Unknown item 'foo'
    E474: 'context' requires a number
    E474: 'ver' number is out of range
    E474: 'algorithm' must be one of: myers, minimal, patience, histogram
    E474: 'filler' does not take a value

Related:

- #31084
- #34661
- #31820
- #14739
- #20107
- fix #18875
  - :get() returns `{ sbr = true, shift = '3' }` (reified-keyset) instead of `{'sbr', 'shift:3'}`
  - Setting via table now works too. `object_as_optval_for` `is_map` now recognizes struct options.
- fix #30296
  - instead of `E474: Invalid argument`, errors now look like:
    ```
    E474: Invalid value 'x', expected one of: single, double: ambiwidth=x
    E474: Unknown item 'foo': diffopt=foo
    E474: 'context' requires a number: diffopt=context:x
    ```

simplify `win_float_parse_option` from #26799.

Problem:
- Lua<=>API roundtrips
- Although we prefer Lua for most business-logic code, doing this
  conversion in C makes sense in this case because:
  1. setting options is a hot path
  2. most of the options logic lives in C
  3. the current arrangement is MORE verbose and requires MORE code

Solution:
Move conversion to a C util.
- nvim_set_option_value passes the raw Object (scalar, Array, or Dict)
  to `object_as_optval_for()` which flattens it to the canonical `:set`
  string and validates the type.
- drop `convert_value_to_vim`, eliminate its roundtrip.

Problem:
`nvim_set_option_value` cannot "update" options similar to `:set opt=`,
`:set opt+=`, etc. The Lua impls of "vim.opt" / "vim.o" have incomplete,
bespoke reimplementations of those operations.

ref #38420

Solution:
- Add `operation` param to `nvim_set_option_value`, which may be "set",
  "append", "prepend", or "remove".
- Use this feature to implement `vim.opt` / `vim.o`.

Problem:
When `to_vim_value[info.metatype](info, value)` is called, a list value
such as `{'eob:a'}` is treated like a map, which generates `1:eob:a`.

Note: commands like `:lua vim.opt.wildmode={'longest:full'}` are not an
issue because only cases harcoded in `key_value_options` have metatype `map`.

Solution:
Check for array type and use the same logic as in array metatypes.