v0.7.7 — fix at-tip proof-digest mismatch via in-place cache resize

v0.7.7 — fix at-tip proof-digest mismatch via in-place cache resize

Root cause: opening a second redb Database handle to resize the read
cache caused incoherent page caches between two mmaps of the same file.

Fix: patched redb (AtomicUsize cache limit + public setter + clear),
new resize_cache() on RedbAVLStorage that resizes on the existing
handle. No second handle, no mmap coherency bug. Also adds a flush
sentinel key to force actual page writes through Durability::Immediate.

Ergo_avltree_rust bumped to 042c830 (tree.reset removed from
generate_proof + old_top_node sync in rollback).

Full Changelog: v0.7.6...v0.7.7