Build a Smaller sccache

Rebuild a concurrent command cache, not merely a HashMap<String, Vec<u8>>.

What are we preserving?

• a short-lived client and resident async server;
• deterministic keys over declared and discovered inputs;
• hit versus compile fallback;
• a separate child-process budget;
• captured output and exit status;
• publication after successful completion;
• runtime-selected storage;
• graceful connection drain.

Stages

1. Cache one fake compiler whose key includes source bytes and arguments.
2. Put the cache behind a local Tokio socket protocol.
3. Execute a real child command and capture declared output files.
4. Add a semaphore-like process permit independent of connections.
5. Implement disk storage through a Storage trait and atomic publication.
6. Return output before awaiting a deliberately slow secondary cache write.
7. Stop listener admission, drain connection tasks, then enforce a deadline.
8. Add simultaneous same-key requests and decide whether to deduplicate them.

Compare the result with production sccache: compiler argument semantics, preprocessing, dependency normalization, archives, many storage systems, distributed toolchains, authentication, multi-level backfill, jobserver interop, platform process handling, statistics, and corruption recovery are the hardening layers still missing.