docs: add LICENSE, SECURITY, CONTRIBUTING, docs index, templates, CODEOWNERS, and rust-toolchain (#113, #114, #115, #116, #117, #119, #120, #121)

[ghzhost]

Summary

This PR establishes complete community health, governance, licensing, developer documentation, and toolchain reproducibility standards for .

Closes #113
Closes #114
Closes #115
Closes #116
Closes #117
Closes #119
Closes #120
Closes #121

Changes

1. **** (No LICENSE file despite every crate declaring license = "Apache-2.0" #114): Adds the official Apache License 2.0 file matching the workspace package license declaration.
2. **** (No SECURITY.md / responsible-disclosure policy #115): Adds responsible disclosure policy, confidential security vulnerability reporting guidelines, and contract scope.
3. **** (No CONTRIBUTING.md documenting build/test/PR conventions #116): Documents onboarding setup, compilation (), testing (), lint/format standards, and PR conventions.
4. **** (No rust-toolchain.toml pinning the Rust version README cites #119): Explicitly pins the Rust toolchain to with target.
5. **** (docs/ has no index linking the four design documents together #113): Adds a central documentation index linking and summarizing all 4 architectural/audit design documents in .
6. **** (No Pull Request template #121): Standardizes PR submissions with checklist for test verification, lint checks, and issue linking.
7. **** (No GitHub Issue templates #120): Adds structured issue templates for Bug Reports and Feature Requests.
8. **** (No CODEOWNERS file for reviewer auto-assignment #117): Configures automated review routing for contracts, scripts, docs, and global workspace.

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