docs: clarify ScanCode relationship wording

[mstykow]

Summary

• clarify public-facing ScanCode relationship wording across README, NOTICE, CLI help/version, and supporting docs
• make non-affiliation explicit on key user-facing surfaces while preserving upstream attribution
• narrow overbroad phrasing in migration, benchmark, architecture, and improvement docs and align xtask version fixtures

Scope and exclusions

• Included:
  • top-level metadata and documentation wording updates in README, NOTICE, Cargo.toml, and CITATION.cff
  • CLI help/version wording plus targeted regression tests
  • supporting docs and xtask fixture alignment for the updated version banner
• Explicit exclusions:
  • no scanner or parser behavior changes
  • no expected-output or golden fixture changes

Intentional differences from Python

• the wording now emphasizes ScanCode-compatible workflows and explicit non-affiliation rather than implying interchangeability or endorsement
• the copyright-detection doc now describes the engine as a Rust reimplementation of the Python reference pipeline rather than a blanket "complete rewrite"

Follow-up work

• Created or intentionally deferred:
  • none