feat(storage): versioned CAS, conditional replace, and prefix delete on StorageAdapter

Three primitives the cluster's object-storage port (RFC-006) needs, on the
engine's existing adapter rather than a parallel store:

- read_text_versioned: content + an opaque backend version token (S3: the
  ETag from GET; local: content sha256 — ETags don't exist on a filesystem).
- write_text_if_match: replace only when the token still matches. S3 maps to
  a conditional put (PutMode::Update / If-Match) — verified against RustFS
  beta.8 through the real object_store 0.12.5 path, no extra builder config
  needed; local compares content then swaps via temp+rename, the same
  single-machine semantics callers had before this trait (safe under their
  own lock protocol, not a cross-process barrier by itself). CAS-lost is
  Ok(None), never silent.
- delete_prefix: recursive + idempotent (local remove_dir_all; S3 list +
  delete, with the non-atomicity documented for crash-retry callers).

Gated S3 coverage: s3_adapter_conditional_writes_contract pins the
conditional-write behavior the cluster ledger will depend on (red if a
backend bump regresses it), and s3_schema_apply_migrates_live_graph closes
the previously-untested schema-apply-on-S3 path before the cluster's schema
executor leans on it. Engine gains the sha2 workspace dep.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Cargo.lock

@@ -4636,6 +4636,7 @@ dependencies = [
  "serde",
  "serde_json",
  "serial_test",
+ "sha2",
  "tempfile",
  "thiserror",
  "time",