TODO: emit ini and round-trip

Goal

Implement model-parser emit ini (or equivalent) so the canonical IR can be lowered back to the ExprTk-style INI authoring shape, and add round-trip tests (INI → IR → INI, and IR → INI → IR) with deterministic output.

Why

• Reduces manual dual maintenance when a team wants both INI and IR in repo.
• Proves the IR is expressive enough for the legacy MPC authoring surface.
• Unlocks “regenerate INI from IR” in libraries without hand-editing two sources.

Scope hints

• New backend module under backends/; no changes to existing backends except shared helpers if deduplication is obvious.
• Normalization rules (ordering, spacing, comments policy) must be documented; byte-identical round-trip may be impossible if INI discards information—define acceptable equivalence (e.g. structural IR equality).

References

• docs/design/model-parser.md roadmap §3.
• docs/design/model-library-and-versioning.md.

Acceptance sketch

• CLI: model-parser emit ini <model.ir.json> [-o out.ini].
• Tests: at least one fixture from current examples/models/*.ini.