fix(master): share bounded rollout cache session

[beinan]

Summary

• add ROLLOUT_CACHE_BYTES to the master with the worker's 2 GiB default and 6:1 index/metadata split
• build one process-wide Lance session and reuse it for records browsing, stats scans, cold observations, retirement, compaction, and index creation
• document a 512 MiB cache budget in the Kubernetes master deployment and add config/session regression coverage

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --all-targets
• cd python && uv run pytest (205 passed, 2 skipped)
• cd python && uv run ruff format --check .
• cd python && uv run ruff check .
• cd python && uv run pyright
• GitHub Actions: 9/9 checks passed