registerDecls swallows its options parameter

## Context

Noted while reviewing #59. `registerDecls(decls, options)` accepts a comptime `options` parameter but discards it (`_ = options;`), while internally passing a hardcoded `.{}` as the options to `createCallback`. The parameter is dead weight in a public signature — callers can pass anything and it silently does nothing.

## Proposal

Either:
- forward `options` (or a per-decl subset of it) to `createCallback` so callers can use arg/return hints (`ArgHint`, returns hints) with `registerDecls`, or
- remove/document the parameter as reserved until there is a concrete design for it.

Ref: `src/register_decls.zig`