Phase 0: measure metadata writes, git transport, and tree serving

Phase 0: measure metadata writes, git transport, and tree serving

Three throwaway spikes under docs/spikes, each with its own harness,
README, and reproduce command. Findings are folded into the roadmap and
its decision log.

- ref-cas: sharded + batched refs sustain 25,637 entity writes/s at 20k
  entities with zero lost writes; a single unsharded ref manages 30/s
  with 22 CAS retries per write and decays quadratically as it fills.
  Metadata refs must be sharded. Unsharded is now off the table.
- smart-http: driving git http-backend as a subprocess is correct (hook
  stderr reaches the client, aborts leave no ref and no orphaned
  children) and pushes 150 MiB/s on compressible data, but a 1 GiB push
  peaks at 1.13 GB of git-child RSS. Phase 1 needs a push size cap and
  a concurrency semaphore, plus chunked-upload spooling.
- tree-serving: sites can be served straight from git trees with no
  export step - 1.2 ms p50 cold, 0.075 ms warm, 170k req/s behind an
  18.5 MiB cache.

Harness bugs caught during the work are recorded in the spike READMEs,
including one that silently dropped 83% of writes while reporting
plausible throughput.