Motivation

The contiguous journal types (fixed::Journal, variable::Journal) and their consumers (merkle::Journaled, Metadata, Queue, qmdb::store::Db, etc.) previously used interior mutability (UpgradableAsyncRwLock, RwLock, AsyncMutex) to allow mutating methods like append, sync, prune, and rewind to take &self. This added locking overhead on every operation and obscured the actual mutability requirements at the type level.

This PR removes the interior-mutability wrappers and changes all mutating methods to take &mut self instead, letting the borrow checker enforce exclusive access statically.

Changes

Persistable trait (storage/src/lib.rs)

commit and sync now require &mut self.

fixed::Journal

• Removed the Inner struct and UpgradableAsyncRwLock wrapper. Fields (journal, size, metadata, pruning_boundary, items_per_blob) are now directly on Journal.
• append, append_many, sync, rewind, prune, clear_to_size now take &mut self.
• size and pruning_boundary are now const fn (no lock acquisition, no async).
• read, bounds, and replay are promoted from test/reader-only helpers to public methods on Journal directly.
• The Reader type becomes a thin borrowed wrapper (&Journal) instead of holding a lock guard. It still implements the Reader trait for use via the Contiguous trait.
• The lock downgrade/upgrade dance in append_many (for syncing full sections mid-append) is replaced by direct &mut self calls.
• Removed the position_to_section helper (inlined).

variable::Journal

Parallel changes to fixed::Journal: removed Inner/UpgradableAsyncRwLock, flattened fields onto Journal, changed mutating methods to &mut self, made size synchronous.

merkle::Journaled

• Removed Inner struct and RwLock wrapper. mem and pruned_to_pos are now direct fields.
• Removed sync_lock: AsyncMutex (no longer needed since sync takes &mut self).
• sync, prune, and related methods take &mut self.
• root is now const fn.
• All self.inner.read().mem / self.inner.get_mut().mem accesses become self.mem.

Metadata

• Removed AsyncMutex around State. state is now a direct field.
• sync takes &mut self.
• Mutable accessors use &mut self.state instead of self.state.get_mut().

Queue

• ack, ack_up_to, size, is_empty are no longer async (they were only async because of the lock).
• Updated shared::Reader and shared::Writer wrappers accordingly.

qmdb::store::Db

• commit, sync, prune take &mut self.
• bounds and size are now const fn (synchronous).

journal::authenticated

• commit and sync take &mut self.
• sync can no longer use try_join! to sync journal and merkle concurrently (since both require &mut self), so they are now sequential.