How an option's raw env/TOML value is coerced to a typed value.
All kinds flow through resolve_scalar. The scalar kinds (BOOL,
BOOL_PRESENCE, INT, FLOAT, STR) are coerced inline by
_coerce_env/_coerce_toml. SHELL_LIST_DELEGATE, SKILLS_DIRS_DELEGATE,
and PTC_DELEGATE defer to a bespoke parser (their semantics — colon-split
Path resolution, comma + recommended/all sentinels, the PTC allowlist —
do not compress into a generic coercion). THEME_DELEGATE is resolved
separately at the top of resolve_scalar and never reaches the inline
coercers. STRUCTURED marks user-defined tables that the scalar resolver
only passes through for display.
