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.
