docs(ilp): add exact scheme spec for ILP (Open Payments)

[dragosp1011]

Description

Adds the protocol specification for the exact scheme on ilp:openpayments.

• specs/schemes/exact/scheme_exact_ilp.md — full spec for exact on ilp:openpayments, covering:
  • Protocol flow (grant pre-approval → incoming payment creation → quote → outgoing payment → verify → no-op settle)
  • PaymentRequirements and PaymentPayload wire format with field definitions
  • Facilitator verification rules
  • Settlement logic (no-op — funds already transferred by the ILP network before /verify is called)
  • SettlementResponse format
  • Security considerations (wallet URL binding, replay attacks, time window enforcement, grant token security, settlement atomicity)
  • Asset scale reference table and network identifier rationale
• specs/schemes/exact/scheme_exact.md — added ILP-specific rules to the scheme overview
  and updated the per-network document index to include scheme_exact_ilp.md.
• specs/CONTRIBUTING.md — updated the directory tree to list scheme_exact_ilp.md.

Relates to #1242

Tests

No code changes — spec documentation update only.

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• I added a changelog fragment for user-facing changes (docs-only changes can skip)