Several Cargo builds at once

Kache is a compiler wrapper, not a build frontend. Keep running cargo. Give each live build its own CARGO_TARGET_DIR (or kache cargo) so Cargo's fingerprints stay isolated.

After a local and remote miss, the wrapper joins a machine-wide flight for the invocation identity — crate name, output kind, and compiler — then takes a memory-weighted permit before the per-key build lock. Hits and passthroughs never wait.

The permit pool size is std::thread::available_parallelism(), with a floor of 1. Unmeasured compiles occupy one slot; unmeasured rustc link invocations occupy two. On Unix, a real compile records the child's peak RSS by crate name. Later invocations of that crate occupy ceil(rss / 512 MiB) slots, clamped to the pool.

If the scheduler directory cannot be used, compilation continues without a permit. Set KACHE_SCHEDULER=0 or [cache] scheduler = false to turn it off.

Two cargo processes that miss the same crate still share one real compile when the first stores an entry the second can restore. Distinct target directories remain required.