docs(flake): annotate KEEP+DEP rationale per standards#102 rule 3

[hyperpolymath]

Per the campaign gate, this flake is LOAD-BEARING — guix.scm uses cargo-build-system with no named inputs (cargo vendors everything); the sealed Containerfile installs only rust cargo. This flake's devShell is the sole source of the rust-overlay rustToolchain and the Rust dev-tool surface (cargo-watch, cargo-edit, cargo-audit, cargo-deny, cargo-fuzz), plus the RSR-template common tools (git, just, nickel, curl, bash, coreutils).

Refs standards#101 standards#102.

Summary

Changes

RSR Quality Checklist

Required

• Tests pass (just test or equivalent)
• Code is formatted (just fmt or equivalent)
• Linter is clean (no new warnings or errors)
• No banned language patterns (no TypeScript, no npm/bun, no Go/Python)
• No unsafe blocks without // SAFETY: comments
• No banned functions (believe_me, unsafeCoerce, Obj.magic, Admitted, sorry)
• SPDX license headers present on all new/modified source files
• No secrets, credentials, or .env files included

As Applicable

• .machine_readable/STATE.a2ml updated (if project state changed)
• .machine_readable/ECOSYSTEM.a2ml updated (if integrations changed)
• .machine_readable/META.a2ml updated (if architectural decisions changed)
• Documentation updated for user-facing changes
• TOPOLOGY.md updated (if architecture changed)
• CHANGELOG or release notes updated
• New dependencies reviewed for license compatibility (PMPL-1.0-or-later / MPL-2.0)
• ABI/FFI changes validated (src/interface/abi/ and src/interface/ffi/ consistent)

Testing

Screenshots