[codex] Add replay capacity envelope metrics

[WaffleBits]

Summary

• add schema-v3 replay capacity-envelope fields for prompt/decode tokens, reserved KV pages, declared prefill/decode/KV capacity, and utilization ratios
• add per-tick admitted prefill token and decoded token counts to make replay aggregates auditable
• regenerate workload replay artifacts and update runtime docs

Validation

• cargo fmt --check
• cargo test
• git diff --check
• cargo run --release -- replay --input fixtures/workload.json --output artifacts/workload-replay.json
• cargo run --release -- replay --input fixtures/workload_pressure.json --output artifacts/workload-pressure-replay.json