Add reader-first Engine quick starts

[konojunya]

Background

The root README led with workspace internals and a long maintainer command list even though both the Rust crate and browser package are public. A new integrator had no root-level install or minimal render example.

Implementation Approach

• Lead with the Engine's user-visible role and interface choice.
• Add copyable Rust and browser render quick starts.
• Summarize deterministic, safe, native/WASM behavior and ownership boundaries.
• Move repository setup, layout corpus checks, snapshots, and release procedures to a new contributor guide.
• Keep detailed package and API references linked rather than duplicating them.

Testing Approach

• cargo test --workspace --locked
• cargo test --doc -p stack-engine --locked
• npm run build:wasm
• npm test
• npm run typecheck
• npm run pack:check
• Compiled and ran the Rust README example in an isolated Cargo project.

Review Perspectives / Points of Concern

Please verify the boundary between consumer-facing guarantees in the README and maintainer-only verification in CONTRIBUTING.