docs(spec): resolve HIGH-severity §7.4↔§7.7 internal consistency (F2/F3/F4/F12)

[dp-web4]

Summary

Resolves 4 of the 5 HIGH-severity findings from the mcp-protocol.md internal-consistency audit (PR #192). These defects made it impossible for a conformant implementation to satisfy §7.4 without depending on the explicitly non-dependable §7.7 WIP section.

• F2: atp_settlement.exchange_rate MUST depended on non-dependable §7.7 → replaced with conditional MUST on block presence + interim conformance note gating internal structure stability on §7.7 finalization
• F3: §7.4 example used scalar bilateral model ({denominator, rate}) that §7.7.1 explicitly rejects → replaced with referent-grounded schema (dual amounts + referent object)
• F4: atp_settlement had no referent slot → now carries caller_currency, responder_currency, caller_amount, responder_amount, referent, and exchange_agreement_ref — forward-compatible with §7.7.3 acceptance payload
• F12: §7.7 subsection conformance status undeterminable → added per-subsection markers distinguishing Normative (§7.7.1, §7.7.4), Normative-draft (§7.7.2, §7.7.3, §7.7.7), and Informative (§7.7.5, §7.7.6)

F8 was already resolved by operator commit 2e1e8b11 (§7.4↔§4.1 wire-field redefinition). This PR continues that spec-fix stream.

Scope discipline

• 1 file modified (web4-standard/core-spec/mcp-protocol.md)
• 0 new files
• No SDK code changes, no test changes, no version bump
• Pure spec refinement — resolves internal contradictions for implementers

Test plan

• Verify §7.4 example JSON is well-formed and consistent with §7.7.3 acceptance schema
• Verify §7.4 normative text no longer contains unconditional MUST on §7.7-dependent fields
• Verify §7.7 subsection status markers are complete (all 7 subsections classified)
• Verify §7.7 intro paragraph references updated §7.4 schema correctly

🤖 Generated with Claude Code

[dp-web4]

APPROVED: Resolves 4 HIGH-severity internal-consistency defects (F2/F3/F4/F12) in mcp-protocol.md §7.4↔§7.7. Before this, a conformant implementation literally could not satisfy §7.4's atp_settlement.exchange_rate MUST without depending on §7.7, which the spec explicitly marks non-dependable WIP — a genuine spec contradiction blocking implementers. The fix is well-scoped: F2 converts the unconditional MUST to a conditional MUST-on-presence with an interim conformance note gating internal structure on §7.7 finalization; F3 replaces the scalar bilateral example (which §7.7.1 rejects) with the referent-grounded schema; F4 gives atp_settlement a referent slot forward-compatible with §7.7.3; F12 adds per-subsection conformance markers (Normative / Normative-draft / Informative). +27 -6, 1 file, pure spec refinement, diff matches description, no drift. This is the base of the remediation stream (#201/#203 build on it) so it merges first.