Fix intra-doc link warnings; add clawsync-fs README and metadata

Doc fixes (20 warnings → 1 benign name-collision warning):
- clawsync-transport/quic.rs: QuicConfig::with_cert doesn't exist → backtick
- clawsync-onion/iblt.rs: bare `insert` link → backtick
- clawsync-core/lib.rs: simd_cdc is feature-gated → backtick
- clawhdf5-onion/annotation.rs: RevisionEntry/BranchEntry are in external
  clawhdf5-format crate, not re-exported → backtick
- clawhdf5-onion/gc.rs: compact_dead_epoch_revisions is private; flush links
  broken → backtick
- clawhdf5-onion/provenance.rs: RevisionEntry from external crate → backtick
- clawhdf5-onion/reader.rs: reconstruct_revision/revision_pages → Self:: prefix
- clawhdf5-onion/writer.rs: REV_FLAG_SNAPSHOT → crate::format:: path;
  reconstruct_revision → Self:: prefix
Remaining warning is `format` module/macro name collision — not a broken link.

clawsync-fs crate metadata:
- Add readme, keywords, categories to Cargo.toml
- Write README.md (CDC protocol diagram, module overview, usage examples)

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

crates/clawhdf5-onion/src/gc.rs

@@ -24,8 +24,8 @@ pub enum GcPolicy {
     /// Computes the dead set using `inner`, marks those entries with
     /// [`EPOCH_DEAD`] in their padding bytes, and returns immediately
     /// without touching `page_data`.  The actual compaction is deferred to
-    /// the next [`OnionFile::flush`] call, which runs
-    /// [`OnionFile::compact_dead_epoch_revisions`] before writing.
+    /// the next `OnionFile::flush` call, which runs
+    /// `OnionFile::compact_dead_epoch_revisions` before writing.
     ///
     /// Use this when you want GC to be non-blocking: the mark pass is
     /// O(revisions) with no I/O; the compaction is amortised into the next
@@ -47,12 +47,12 @@ impl OnionFile {
     ///
     /// For all policies except [`GcPolicy::EpochFlip`]:
     /// - The `RevisionIndex` is updated immediately.
-    /// - Page data is compacted in-memory; call [`flush`] to persist.
+    /// - Page data is compacted in-memory; call `OnionFile::flush` to persist.
     ///
     /// For [`GcPolicy::EpochFlip`]:
     /// - Dead revisions are *marked* with [`EPOCH_DEAD`] in O(revisions).
     /// - Page data is **not** touched; compaction is deferred to the next
-    ///   [`flush`] call.  This makes the GC call itself non-blocking.
+    ///   `OnionFile::flush` call.  This makes the GC call itself non-blocking.
     ///
     /// **Note:** Immediate GC is irreversible.  Epoch-flip GC can be
     /// cancelled by calling `flush_wal()` without `flush()`, but only