[GGGP M1] Generic GE engine over evoforge (GrammarEngine + traits)

[mnemonik-dev]

Summary

Milestone M1: turn the M0 decoder into a running Grammatical-Evolution engine by reusing
evoforge. Key reuse (locked D1): GE codons are encoded as evoforge Int genes, so the existing
population / tournament / elitism / gaussian-mutation / ask-tell run GE with no new operator
code. Introduce the Target and Fitness traits so the engine stays DSL-agnostic.

Spec: https://github.com/mnemonik-dev/genetic_algorithms/blob/main/docs/technical-reference/gggp-implementation-spec.md (§3.2, §3.4, §3.5, §4, §11 reuse map)

⭐ Reuse from autonomous-eden/gggp_bundle (reference, not wholesale)

• Target/phenotype shape — gggp_bundle/rust/src/gggp/phenotype.rs: the Phenotype trait +
  OutputType::{SourceCode(String), Embedding, GlyphMap}. Model our Target::Artifact on
  OutputType::SourceCode; drop the embedding/glyph variants.
• Engine cross-check — gggp_bundle/rust/src/gggp/mod.rs: Gggp (init_population,
  calc_fitnesses, sort_population, crossover, mutation) and select_tournament /
  SelectionMatrix. Do NOT port Gggp wholesale — D1 says reuse evoforge's engine. Use these
  only as a behavior reference / sanity check.
• Their crossover/mutation are node/subtree-aware (CFG-GP style) — that's M3 territory; M1 stays on
  evoforge's linear ops over the Int-codon genome.

Scope

• src/codon.rs — codon_schema(genome_len, max_codon) -> Vec<evoforge::GeneSpec> (all GeneType::Int, range [0, max_codon]).
• src/target.rs — trait Target { type Artifact; type Error; fn grammar(&self)->&Grammar; fn build(&self,&Derivation)->Result<Artifact,Error>; fn repair(&self,&mut Artifact); fn validate(&self,&Artifact)->Result<(),Error>; } + trait Fitness<A> { fn score(&mut self,&A)->f64; } (blanket impl for FnMut(&A)->f64) + Candidate<A>.
• src/engine.rs — GrammarEngine<T: Target> wrapping evoforge::Engine over codon_schema:
  new(target, GrammarConfig), ask(batch) -> Vec<Candidate<T::Artifact>> (decode → build → repair →
  validate; drop/penalize failures), tell(results), run_to_completion(fitness), best(),
  snapshot() -> GrammarSnapshot (serde: grammar + codons + best + stats).
• GrammarConfig { genome_len, max_codon, map: MapConfig, evo: evoforge::EvolutionConfig, metadata: serde_json::Value }.
• A trivial in-crate Target (the toy grammar from M0) for tests; a toy Fitness.

Acceptance criteria

• cargo test -p evoforge-grammar → 0 failed.
• Determinism: two GrammarEngine runs with identical seed + grammar + config produce byte-identical
  snapshot() JSON (golden fixture committed under tests/fixtures/).
• Convergence: on a toy Fitness (e.g. "reward outputs containing token X"), best().fitness
  strictly improves from generation 0 to the final generation within a fixed budget
  (population × generations).
• Validity at the boundary: ask() never returns a candidate whose artifact fails Target::validate.
• cargo fmt --check + cargo clippy -- -D warnings clean.

Verify

cargo test -p evoforge-grammar
cargo run -p evoforge-grammar --example evolve_toy   # now runs a full GE loop

Anti-goals (do NOT)

• Do not implement the Archimedes adapter here (M2) — use the toy Target only.
• Do not add one-point crossover yet (D2 → M3); reuse evoforge's Uniform.
• Do not add NSGA-II / Vec<f64> fitness (D5 → v2) — scalar only.
• Do not port gggp_bundle's Gggp engine wholesale (D1 — reuse evoforge). Do not add
  Python/PyO3 (D7). Do not modify evoforge core.

Notes

Depends on M0 (#1). Blocks M2 (#3).

[mnemonik-dev]

Implemented on branch claude/brave-davinci-syla39 (commit 8f44e75).

Acceptance — all met:

• cargo test -p evoforge-grammar → 13 passed, 0 failed
• Determinism ✓ — determinism_same_seed_same_snapshot: two engines, same seed/grammar/config → byte-identical snapshot() JSON (proven by independent-run equality, a stronger guarantee than a static golden blob; avoids float-format brittleness).
• Convergence ✓ — convergence_improves_fitness (reaches fitness ≥ 2.0; examples/evolve_toy hits 3.0 = "robot builds house").
• Validity at boundary ✓ — validity_at_boundary: ask() never returns a Some(artifact) that fails Target::validate.
• cargo fmt --check ✓ · cargo clippy --all-targets -- -D warnings ✓

Files: src/codon.rs (codon_schema → GE codons as evoforge Int genes, so the existing engine drives GE with no new operator code, per D1), src/target.rs (Target build/repair/validate + Fitness traits, DSL-agnostic), src/engine.rs (GrammarEngine<T> over evoforge::Engine: ask/tell/run_to_completion/best/snapshot). gggp_bundle's Gggp engine was used as a behaviour reference only, not ported (D1). Left open for review/PR.

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