feat: XLS-93d: Token-Enabled Payment Channels

[dangell7]

High Level Overview of Change

Token-Enabled Payment Channels (XLS-93d). Extends payment channels to IOUs and MPTs the same way XLS-85 extended escrows, reusing the issuer opt-in flags and the locked-amount accounting introduced there.

This is the XRPLF-branch move of #5448. PaymentChannelClawback is split into a stacked PR that merges into this branch; it is part of the same featureTokenPaychan amendment, so it lands before activation.

Context of Change

Payment channels were the last XRP-only locking primitive. This mirrors XLS-85: issuers opt in with lsfAllowTrustLineLocking / lsfMPTCanEscrow, locked value is tracked in sfLockedAmount, and the shared lock/unlock helpers are reused so escrow and payment channels behave identically under the same token conditions.

API Impact

• Public API: New feature (new fields on the PaymentChannel transactions and ledger object)
• libxrpl change

[github-actions]

This PR has conflicts, please resolve them in order for the PR to be reviewed.

[xrplf-ai-reviewer]

Clean, thoughtful implementation that extends existing patterns well. No issues to flag.

[github-actions]

All conflicts have been resolved. Assigned reviewers can now start or resume their review.

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
include/xrpl/protocol/PayChan.h	87.9%	4 Missing ⚠️
src/xrpld/rpc/handlers/account/GatewayBalances.cpp	66.7%	4 Missing ⚠️
...ansactors/payment_channel/PaymentChannelCreate.cpp	96.3%	2 Missing ⚠️
...nclude/xrpl/ledger/helpers/PaymentChannelHelpers.h	91.7%	1 Missing ⚠️
...c/libxrpl/ledger/helpers/PaymentChannelHelpers.cpp	96.3%	1 Missing ⚠️
...ransactors/payment_channel/PaymentChannelClaim.cpp	98.5%	1 Missing ⚠️
...transactors/payment_channel/PaymentChannelFund.cpp	98.2%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[xrplf-ai-reviewer]

Looks solid. Ready to merge.