[codex] migrate generator to direct g4 input

[tinovyatkin]

Summary

• generate the Gremlin lexer and parser directly from the official combined Gremlin.g4
• migrate the crate to the new generated GremlinParser module and ParsedFile API
• pin runtime and generator provenance to ophi-dev/antlr-rust-runtime@c31ce00172d7a38b14dc991878caab810c40148f
• update regeneration and user documentation for the source-only workflow

Why

This is the gremlin-rs integration branch for ophi-dev/antlr-rust-runtime#141. It exercises a large combined grammar and the intentional generated parser naming change against the exact Phase C runtime candidate.

Impact

Parser regeneration no longer requires Java, the ANTLR tool jar, or .interp metadata. Existing parse entry points retain their behavior while returning the runtime's paired ParsedFile representation.

Validation

• cargo test --locked --all-features (unit, integration, and doctests)
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• direct regeneration from grammar/Gremlin.g4 produced no drift at the final runtime pin

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.