docs: systematize feature requirements

[dapi]

Summary

• define a compact, systems-engineering requirement taxonomy for Feature Flow
• add bidirectional traceability from requirements to exact implementation surfaces, checks, evidence, and CI
• preserve existing mnemonics and document incremental migration
• support source-template delivery without mutating its managed downstream snapshot

Validation

• ruby tools/validate-priming-manifests.rb template/memory-bank
• memory-bank-cli lint --scope-root template/memory-bank --entrypoint template/memory-bank/README.md
• memory-bank-cli doctor --profile template
• git diff --check
• codex review --uncommitted

Closes #98