feat(flags): ramp the Rust cache writer on the invalidation path to 100%

The shadow compare ran at 100% from 2026-09-04 with zero mismatches, so the Rust build logic is validated against production input in both regions. The flags-cache-kafka-dual-write flag pins four internal projects at 100% and runs a general ramp at 1% since 2026-09-07. Continue the ramp from 1% to 100%, with a hold window at each step.

Constraints on the ramp:

• One flag drives both regions. Before the 1% step, prod-EU never ran persist_flags_cache, so early hold windows watch EU write health first.
• Each ramped team leaves the shadow compare and falls back to the weaker Python verifier. The hold windows price that trade.
• Watch builds_total{result="failure"}, the DLQ, and the verifier repair rate. A repair loop on a ramped team means a wrong write that the failure counters missed.

At 100%, the refresh sweep and the verifier still write with Python. That is out of scope here. See the separate issue on a single Rust writer.