Move the settlement-failed error to the stellar spec.

Author: marcelosalloum

specs/core/draft-httpauth-payment-00.md

@@ -576,7 +576,6 @@ below, and the canonical base URI for problem types is
 | `method-unsupported` | 400 | Method not accepted |
 | `malformed-credential` | 402 | Invalid credential format |
 | `invalid-challenge` | 402 | Challenge ID unknown, expired, or already used |
-| `settlement-failed` | 402 | Credential valid but settlement failed |
 
 ## Retry Behavior
 

specs/methods/stellar/draft-stellar-charge-00.md

@@ -415,6 +415,20 @@ MUST treat this as a server error (HTTP 5xx) rather than a
 6. `timeBounds.maxTime` MUST NOT exceed the `expires` timestamp from the
    challenge.
 
+# Error Codes {#error-codes}
+
+This specification defines the following additional error code beyond those
+in {{I-D.httpauth-payment}}:
+
+| Code | HTTP | Description |
+|------|------|-------------|
+| `settlement-failed` | 402 | Credential valid but on-chain settlement failed |
+
+Servers MUST return `settlement-failed` when a credential passes
+verification but the Stellar transaction fails on-chain (e.g., insufficient
+funds or sequence number conflict). This is distinct from
+`verification-failed`, which indicates the credential failed validation checks.
+
 # Settlement Procedure {#settlement}
 
 ## Sponsored Flow
@@ -437,7 +451,7 @@ MUST treat this as a server error (HTTP 5xx) rather than a
 
 7. On `SUCCESS`, return a receipt per {{receipt}}. On `FAILED`, the server
    MUST return a `settlement-failed` error per
-   {{I-D.httpauth-payment}}. The credential was valid but the on-chain
+   {{error-codes}}. The credential was valid but the on-chain
    transaction failed (e.g., insufficient funds or sequence number
    conflict).
 
@@ -453,7 +467,7 @@ MUST treat this as a server error (HTTP 5xx) rather than a
 
 4. On `SUCCESS`, return a receipt per {{receipt}}. On `FAILED`, the server
    MUST return a `settlement-failed` error per
-   {{I-D.httpauth-payment}}. The credential was valid but the on-chain
+   {{error-codes}}. The credential was valid but the on-chain
    transaction failed (e.g., insufficient funds or sequence number
    conflict).