Motivation
The block cache currently admits every full page handed to BlockCache::put() (deltas are already skipped). To tune memory usage and protect against caching low-value pages, operators need a knob to admit only a percentage of clean pages.

Approach

• Add a new startup parameter to src/mongo/db/modules/atlas/src/disagg_storage/server_parameters.idl (e.g. disaggBlockCacheCleanPagePercentage, double/int 0–100), alongside the existing disaggBlockCacheSizeBytes / disaggBlockCacheNumShards knobs.
• Apply the admission decision in BlockCache::put() (pali_block_cache.cpp) — admit a clean page only if it falls within the configured percentage (deterministic sampling on the cache key, not RNG, to stay reproducible).
• Define behavior at the bounds (0 = admit no clean pages, 100 = current behavior).

Definition of Done

• New knob exists, is documented in the IDL, and defaults to current behavior (no regression when unset).
• BlockCache::put() honors the percentage for clean pages.
• Unit test in pali_test.cpp covers 0%, 100%, and an intermediate value.