feat(rust/volume_server): add compatibility launcher and migration plan

rust/volume_server/DEV_PLAN.md

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+# Rust Volume Server Rewrite Dev Plan
+
+## Goal
+Build a Rust implementation of SeaweedFS volume server that is behavior-compatible with the current Go implementation and can pass the existing integration suites under `/Users/chris/dev/seaweedfs2/test/volume_server/http` and `/Users/chris/dev/seaweedfs2/test/volume_server/grpc`.
+
+## Compatibility Target
+- CLI compatibility for volume-server startup flags used by integration harness.
+- HTTP and gRPC behavioral parity for tested paths.
+- Drop-in process integration with current Go master in transition phases.
+
+## Phases
+
+### Phase 0: Bootstrap and Harness Integration
+- [x] Add Rust volume-server crate.
+- [x] Implement Rust launcher that can run as a volume-server process entrypoint.
+- [x] Add integration harness switches so tests can run with:
+  - Go master + Go volume (default)
+  - Go master + Rust volume (`VOLUME_SERVER_IMPL=rust` or `VOLUME_SERVER_BINARY=...`)
+- [ ] Add CI smoke coverage for Rust volume-server mode.
+
+### Phase 1: Native Rust Control Plane Skeleton
+- [ ] Native Rust HTTP server with admin endpoints:
+  - [ ] `GET /status`
+  - [ ] `GET /healthz`
+  - [ ] static/UI endpoints used by tests
+- [ ] Native Rust gRPC server with basic lifecycle/state RPCs:
+  - [ ] `GetState`, `SetState`, `VolumeServerStatus`, `Ping`, `VolumeServerLeave`
+- [ ] Flag/config parser parity for currently exercised startup options.
+
+### Phase 2: Native Data Path (HTTP + core gRPC)
+- [ ] HTTP read/write/delete parity:
+  - [ ] path variants, conditional headers, ranges, auth, throttling
+  - [ ] chunk manifest read/delete behavior
+  - [ ] image and compression transform branches
+- [ ] gRPC data RPC parity:
+  - [ ] `ReadNeedleBlob`, `ReadNeedleMeta`, `WriteNeedleBlob`
+  - [ ] `BatchDelete`, `ReadAllNeedles`
+  - [ ] copy/receive/sync baseline
+
+### Phase 3: Advanced gRPC Surface
+- [ ] Vacuum RPC family.
+- [ ] Tail sender/receiver.
+- [ ] Erasure coding family.
+- [ ] Tiering/remote fetch family.
+- [ ] Query/Scrub family.
+
+### Phase 4: Hardening and Cutover
+- [ ] Determinism/flake hardening in integration runtime.
+- [ ] Performance and resource-baseline checks versus Go.
+- [ ] Optional dual-run diff tooling for payload/header parity.
+- [ ] Default harness/CI mode switch to Rust volume server once parity threshold is met.
+
+## Integration Test Mapping
+- HTTP suite: `/Users/chris/dev/seaweedfs2/test/volume_server/http`
+- gRPC suite: `/Users/chris/dev/seaweedfs2/test/volume_server/grpc`
+- Harness: `/Users/chris/dev/seaweedfs2/test/volume_server/framework`
+
+## Progress Log
+- Date: 2026-02-15
+- Change: Created Rust volume-server crate (`weed-volume-rs`) as compatibility launcher and wired harness binary selection (`VOLUME_SERVER_IMPL`/`VOLUME_SERVER_BINARY`).
+- Validation: Rust-mode conformance smoke execution pending CI and local subset runs.