P0-S2b: Merkle adapter + transaction overlay and commit

[mconcat]

P0-S2b: Merkle adapter + transaction overlay and commit

Background

We need persistence to a Merkle-backed KV so contract writes survive across executions and produce deterministic state roots.

Goal

Implement a minimal backend adapter and per-execution overlay with deterministic batch commit.

Design

• Backend adapter: get/put/del/commit(batch)->root with path-to-key mapping (e.g., home:contract:data.txt).
• Overlay: execution-scoped map for writes/deletes/renames; reads check overlay first.
• Commit: on success, commit sorted batch (lexicographic by key) and expose pre/post roots; on failure, discard overlay.

Constraints

• Deterministic ordering and results across nodes; no clocks/randomness.
• Directory listing must not depend on KV range scans in P0; rely on logical directory markers.

Acceptance Criteria

• Writes are visible to subsequent executions; state root changed deterministically.
• Unit tests cover commit ordering and rollback on failure.

References

• Epic: P0: WASI File I/O Streams over VFS with Merkle-backed Persistence #79; Stat-by-fd: P0-S2a: Stat-by-fd helper and accurate sizes #83; PR: Feat/vfs #78