Cache: WIP (1/5) Add content-addressed local inference caches

[ErlisLushtaku]

Description

Note

WIP: replacement cache stack on top of #118. Not final and may still change.

Adds content-addressed local SQLite caches for model completions and judge outputs.

Cache layout:

{store_root}/completions/{task}/{provider}/{model}/{descriptor_hash}/
├── metadata.json
└── completions.db

{store_root}/judgements/{task}/{provider}/{model}/{descriptor_hash}/
├── metadata.json
└── judgements.db

metadata.json stores the validated model descriptor. The databases use these schemas:

completions(
  input_hash TEXT PRIMARY KEY,
  input_text TEXT NOT NULL,
  completion TEXT NOT NULL,
  benchmark TEXT NOT NULL,
  instruction_id TEXT NOT NULL,
  model TEXT NOT NULL,
  pushed_by TEXT NOT NULL,
  pushed_at TEXT NOT NULL,
  run_id TEXT NOT NULL
)

judgements(
  input_hash TEXT PRIMARY KEY,
  judge_input TEXT NOT NULL,
  judge_completion TEXT NOT NULL,
  benchmark TEXT NOT NULL,
  instruction_id TEXT NOT NULL,
  model_a TEXT NOT NULL,
  model_b TEXT NOT NULL,
  judge TEXT NOT NULL,
  top_logprobs TEXT,
  orientation TEXT,
  pushed_by TEXT NOT NULL,
  pushed_at TEXT NOT NULL,
  run_id TEXT NOT NULL
)

top_logprobs is new relative to the earlier draft so judgements can restore the first-token logprobs used by #118 parsers.

This is stacked on #118.