feat: add machine-readable JSON continuation envelopes

[yearth]

Summary

• add --json-envelope for structured { data, meta } output
• expose more, complete, and past_end states with an exact nextOffset for safe continuation
• support selector, row, table, and locate output while preserving the existing top-level array returned by --json
• reject incompatible output modes before source I/O and document the continuation workflow

Testing

• bun test — 145 passed
• bun test test/cli.test.ts — 64 passed
• bunx tsc --noEmit
• bun run format
• bun run build
• compiled two-page continuation smoke: more → complete, with no overlap or gaps

[yusukebe]

I think nextOffset should be next_offset because other names use snake_case.

[yearth]

Thanks for pointing this out. This was an oversight in my review of the repository’s JSON naming convention. I’ve renamed the public field to next_offset and checked the rest of this PR for similar JSON naming issues; I didn’t find any others.

[yusukebe]

LGTM!

[yusukebe]

@yearth

Thank you for the PR and the quick fix. This is a good feature!