fix(psp-1)

- Number all sections/subsections; update cross-references accordingly.
- Remove residual "(normative)" labels; mark non-binding parts "(Informative)" only.
- Add "References" section
- Remove Appendix D (reason codes) from PSP-1; add pointer to PSP-2 for reason codes and receipt schemas.
- Security: add short "Threat Model" subsection; reference TAP for acceptance controls (exposure mode constraints, adapter assurance).

Author: CMCDragonkai