docs: v2.25.0 — extract and formally reference every TC18 SHOULD/MAY clause

[SoundMatt]

Summary

• Mirrors c-RCP's own SHOULD/MAY audit (see its PR rewrite(phase6): batch 9 — MDIO/PWM catalog re-derivation #162 / ROADMAP milestone 116).
• All 51 non-legal SHOULD (10) / MAY (41) occurrences in TC18 individually classified.
• 6 already-implemented optional capabilities got a real tc18 citation added to their existing requirement entry: REQ-E2E-011, REQ-PWR-001, REQ-LIFECYCLE-001, REQ-SPI-001, REQ-ADC-002, REQ-L2-007.
• The non-testable lines are individually cited in new docs/TC18-NON-NORMATIVE-CLAUSES.md.
• Honestly flags 4 MAY-described capabilities this repo does not appear to distinctly implement yet (Timed presentation-time-too-far/gPTP admission, the EP_USED bit, Triggered requests' field-level encode/decode, integrated-PHY-via-MDIO) for their own dedicated follow-up, rather than papering over them with a citation.
• No code behavior changed. A fuller MUST-clause citation backfill (this repo had only 4/376 requirements citing TC18 before this session) remains separate, larger future work.

Test plan

• .fusa-reqs.json valid JSON, surgical diff
• Full clean build + ctest: 53/53 passed
• cpfusa check (fresh v0.18.0 build matching CI's exact pin): 0 errors
• cpfusa trace --req-coverage 100 --sec-tested 100: 376/376 traced and tested