Module env_config 

Test-mode GUC snapshot — frozen at engine init.

Hot paths in the engine read fields directly via engine.env_cfg().<field>. In production EnvConfig is Default::default() so every field is false / None / Auto — the optimiser can const-fold every gate.

Adding a new GUC:

1. Add a field here + default + builder method.
2. Add the read site (engine.env_cfg().<field>) at one acceptor in the engine.
3. Add a tests/env_cfg_<name>.rs unit pin.
4. Record all three in xtests/sigil/test-mode-gucs.md.

Structs

EnvConfig

Frozen snapshot of SPG_TEST_* env vars + programmatic overrides.

EnvConfigBuilder

Builder for EnvConfig. One method per field; symmetric with the env vars in from_env.

Enums

ComputeQueryId

Semantics of the compute_query_id knob.