Rewrite the cache on a fixed slab without per-entry allocation

cache.cpp keeps its C API but replaces uthash and the two mallocs per
entry with one slab of nodes holding their keys inline, linked in
last-use order, and an open-addressing index of node ids that hashes
through the slab with a per-cache random seed. Both blocks are reserved
by cache_create() and touched only as entries are used, so a large,
mostly empty cache costs no resident memory.

Semantics are unchanged: an insert evicts the least recently used entry
once the cache holds capacity entries (the AEAD-2022 session table's
4095/4096 boundary), lookups refresh recency and age, cache_clear()
drops stale entries, and free_cb may re-enter the cache. Re-inserting a
key now replaces its data instead of shadowing it. cache_create_keyed()
sizes keys per cache; the AEAD-2022 salt pool and session table use it.

test_cache_cpp covers the boundary, ordering, replacement, ageing,
re-entrant callbacks and limits; alloc_steady_cache requires zero
allocations across 100,000 inserts on a full cache. AEAD-2022 ss-server
drops from 84 to 80 allocations per relayed connection.

scripts/no_heap_baseline.txt

@@ -5,7 +5,6 @@
 src/aead.cpp 4
 src/aead2022.cpp 3
 src/aead2022_udp.cpp 5
-src/cache.cpp 3
 src/crypto.cpp 4
 src/ipset.cpp 3
 src/jconf.cpp 16