Make Rust the single writer for the HyperCache estate

[!WARNING]
AI draft — James has not edited this yet.

At 100% of the invalidation ramp, Python still writes about 126,800 flags.json entries per day through the refresh sweep and the verifier. The Rust edit path writes about 24. update_flags_cache stays bound as update_fn on FLAGS_HYPERCACHE_MANAGEMENT_CONFIG, independent of routing. So two writers is the steady state, not a transition, and a team's flags.json alternates authors on a schedule.

Desired outcome: Rust owns the edit path, the refresh sweep, and the verifier for every HyperCache instance. Python keeps no write path and, at the end, no build code.

Python writes these caches today:

• flags.json — the refresh sweep and the verifier, plus the edit path for teams outside the ramp
• team_metadata — Rust already owns its read exclusively
• flags_with_cohorts.json — the definitions cache, with its Redis-ZSET-plus-Celery rebuild subsystem
• array/config.json
• surveys, gateway_credential, llm_gateway_policy, llm_prompts — no Rust reader today