Separate requested Cell releases from pressure shedding budget

[forhappy]

Summary

• Give explicit idle-Cell releases their own bounded 32-in-flight, 32-completions-per-second allowance, while retaining the existing two-per-second automatic pressure-shedding budget.
• Keep generation checks, transfer preflight, authoritative release, and bounded admission unchanged.
• Add a budget isolation regression and document the distinction.

Verification

• Focused budget tests: passed.
• Five exact idle-release lifecycle tests: passed.
• Pressure-eviction test: passed.
• cargo clippy -p cellule-runtime --all-targets --locked -- -D warnings: passed.
• cargo fmt --all --check, documentation links, and Rust snippets: passed.

This raises an admission ceiling for cold repository churn; it is not a measured sustainable object-store rate or a 10,000-repository qualification.