Memoize env-keyed subprocess probes through one helper: composer, python and ruby each define the same env key and drifted on when an answer is kept #1888

[agent] Filed by the scheduled architecture audit (audit-core). Register: register.

Kind: refactor. Register ID: A32.

Problem

Three crawlers memoize a subprocess answer (composer config, the Python site query, gem env), keyed on the sorted process environment plus the cwd. The key function is byte-identical in all three:

let mut vars: Vec<_> = std::env::vars_os().collect();
vars.sort();
(vars, std::env::current_dir().ok())

• composer: EnvKeyedMemo + env_key, composer_crawler.rs L477-L550.`` It is a single slot, and it keeps an answer only if the environment is unchanged after the subprocess returns.
• python: KeyedMemo<K, V> + site_query_key, python_crawler.rs L2765-L2817.```` It is a single slot and stores every answer (let value = run()?; *lock() = Some((key, value.clone()))). It has no env-unchanged check, so an answer computed while another thread changed the environment is filed under the old key.
• ruby: GemEnvMemo (a HashMap of OnceCells) + gem_env_key, ruby_crawler.rs L1012-L1023 and L1625-L1650.`````` It keeps every key it has ever seen, never evicts, single-flights per key, and has composer's env-unchanged check.

The drift (python's missing env-unchanged check) comes from reading the code; I did not execute it. It only matters when tests or the CLI change env concurrently.

Proposed change

• Add one utils::process::EnvKeyedMemo<V> with an env_key(). Use a single slot and composer's rule: keep an answer only when the env is unchanged after the run.
• Composer and python use it directly.
• Ruby's async single-flight can wrap the same key type and keep-rule. Alternatively, ruby's map can become the single slot, since no caller needs more than one environment at a time.
• Delete the three key functions and the two memo structs.

Size

3 files plus utils/process.rs, about −60 lines.

Acceptance criteria

• Behavior is unchanged. Composer's get_or_run_keyed tests, python's memo test (python_crawler.rs ~L8126) and ruby's memoize_gem_env_homes tests must stay green and move to the shared helper.
• One test covers the keep rule: an answer computed while the env changed is not cached.