Red phase for #51: covers strict cache_max_bytes parsing (invalid
explicit values abort naming key and value), the computed default of
max(75% of free space, 500 MiB) via an injectable free-space probe,
explicit-value-no-floor, zero-disables-cache, size accounting over
source blobs and variants, LRU eviction under the limit, the
multi-referenced blob case, write-pressure and periodic eviction
triggers, and startup reconciliation. Minimal API skeletons keep the
tree compiling and lint-clean; only the new tests fail.
