fix(dev): add port 9000 preflight, recommend Nix devshell, add DEV_SETUP.md (fixes #11)

[chiang21fcb]

Problem and value

Fixes #11: three concrete improvements to make the build setup more hermetic and contributor-friendly.

Changes

1. Port 9000 preflight check in justfile dev-store: verifies port 9000 is free before starting rustfs. Fails fast with a clear error message and platform-specific fix instructions when the port is in use (common with Zscaler and other corporate tools on macOS).

2. README now recommends nix develop as the canonical path (it already pins all tools), with a link to docs/DEV_SETUP.md for the bring-your-own-tools fallback.

3. New docs/DEV_SETUP.md: per-platform tool installation instructions for macOS (Apple Silicon), Ubuntu/Debian, and Arch Linux, plus a verification checklist.

Scope

• Deliberately did not change: macOS podman socket path detection in dev-store (the setsid / /run/user/$UID path). That is a larger change best addressed separately.
• No Rust or web code changes

Stability impact

• No compatibility risk: the port preflight only runs in dev-store, which is a local development recipe
• No runtime impact: changes are to documentation and development tooling only

Tests run

• The port preflight check was manually tested on macOS with port 9000 both free and occupied
• just test not run locally (requires Rust toolchain and protoc)
• No code changes to test

Visual evidence

Not applicable.

Checklist

• I used a minimal focused change
• No generated artifacts changed
• No secrets, private content, or customer data in fixtures