feat(errors): include byte offset and path in Phase 1 parse errors

[membphis]

Problem

qjd_parse currently returns an opaque QJD_PARSE_ERROR on malformed JSON with no position information. Callers cannot tell the user where the JSON is broken, making debugging difficult.

Proposal

Extend the error surface to include:

1. Byte offset of the first invalid byte detected during Phase 1 (structural scan).
2. Optionally a path context string (e.g. "[42]" for an error inside the 43rd array element) — this is harder and may be deferred further.

Minimal viable version: expose the byte offset only.

API sketch

// New field on the existing error type, or a companion query function:
size_t qjd_parse_error_offset(const qjd_doc *doc);

The offset is only meaningful when qjd_parse returns QJD_PARSE_ERROR; it is SIZE_MAX otherwise.

Implementation notes

• validate_brackets in src/scan/mod.rs already returns the failing usize offset; it is currently discarded.
• Store the error offset in Document and expose it via the new FFI call.
• Update the LuaJIT wrapper to surface the offset in the error message string: "JSON parse error at byte 1234".
• Lua busted tests: verify offset is correct for truncated input, unbalanced braces, and bare invalid bytes.