feat: add Rust environment

[sanketsudake]

What

Adds a new Rust language environment (rust-env / rust-builder), closing #81.

Design

The builder compiles each function into a complete native HTTP server binary (axum 0.8 / tokio).
The runtime image runs a small Rust supervisor that implements the env contract on :8888, spawns the function binary once at specialize, and reverse-proxies all requests to it over a pooled keep-alive connection — no per-request fork, no dynamic library loading (the FFI approach attempted in #81 is unsound because Rust has no stable ABI).

┌─ rust-env pod ───────────────────────────────┐
│  supervisor :8888                            │
│   ├ GET  /healthz                            │
│   ├ POST /specialize, /v2/specialize         │
│   └ /*  ── streaming reverse proxy ──▶ child │
│  function binary 127.0.0.1:8889              │
└──────────────────────────────────────────────┘

Function authoring

• Single-file mode (--src hello.rs): one .rs file with pub async fn handler (any axum handler signature); extra sibling module files supported. The builder wraps it in a pre-compiled template crate, so builds only recompile the handler.
• Cargo project mode (--src project.zip): any binary crate serving HTTP on $FISSION_RUNTIME_PORT — bring your own framework (axum/actix/rocket/warp). Multi-[[bin]] projects deploy all binaries and --entrypoint selects one at specialize.
• fission-rust SDK crate (in-repo, vendored into the builder): fission_rust::serve(handler).

Contract notes

• v1 /specialize loads from /userfunc/user; v2 handles file or directory filepath + functionName, with empty-filepath fallback like the binary env.
• Bad deploy packages return 400 (parity with binary/server.go); pre-specialize requests return 500 "Container not specialized".
• If the function process exits, the supervisor exits so Fission replaces the pod.
• Builds inside one builder pod are serialized with flock; DEPLOY_PKG is created up front (the exact failure from Rust environment support #81).

Testing

• cargo test — 12 unit tests (binary resolution, specialize contract, port resolution)
• rust/test/local_test.sh — cluster-free contract test (specialize + proxy, both modes)
• rust/tests/test_rust_env.sh — kind e2e: single-file (poolmgr + newdeploy) and Cargo project (POST echo)
• Verified locally: both images build; builder runs in all three modes (single-file, multi-module, project); runtime specializes and proxies each artifact

Release

envconfig.json starts at 1.0.0; first merge publishes rust-env/rust-builder via the GHCR gate. The fission.io site needs a sync for the new environment after merge.

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