Prepare Stack Engine 0.9.1

[konojunya]

Background

The arrow terminal clearance correction is merged on main and needs immutable Rust and browser package artifacts for downstream consumers.

Implementation Approach

• Bump the Rust engine, WASM adapter, and npm package metadata to 0.9.1.
• Refresh reviewed SVG metadata snapshots for the new engine version.
• Add release notes covering the geometry change, compatibility boundary, and rollback pin.

Testing Approach

• cargo test --workspace --locked
• Rust 1.85 tests and clippy
• Stable clippy and rustdoc warnings
• Formatter and engine conformance suites
• Native/browser WASM parity tests
• SVG validation and reviewed snapshots
• Release-mode layout and language-intelligence performance gates
• 95% coverage gates
• Cargo publish dry-run and npm package check

Review Perspectives / Points of Concern

• Public APIs and theme contracts remain unchanged.
• Exact SVG bytes intentionally change for routes affected by arrow terminal clearance.
• Consumers can roll back by pinning Engine 0.9.0.