refactor(swaps): rename preimage tag NArk → Arkade for cross-SDK interop

NArk is the .NET SDK's internal shorthand; using it in the on-the-wire
derivation tag locks the scheme to one specific SDK implementation. The
preimage is rooted in BIP-340 signing of a fixed message — there's
nothing about it that's .NET-specific. Renaming the tag to "Arkade-
Boltz-Preimage-v1" lets any Arkade SDK (ts-sdk, go-sdk, rust-sdk, NArk)
implement the same scheme byte-for-byte and recover swaps that another
SDK created, which is the entire point of having a documented spec for
the message construction.

Arkade is the protocol brand per CLAUDE.md; Boltz is the provider
(future Loop/Submarine/etc. would have their own tag); -v1 stays as
the version hook.

Pre-merge: nothing has shipped under the old tag yet, so this is a
free rename — no v1/v2 fallback needed.

docs/articles/swaps.md updated to make the cross-SDK interop intent
explicit so any future implementer knows the tag is canonical and
not an SDK-specific opaque value.

Author: Kukks

NArk.Swaps/Services/SwapsManagementService.cs

@@ -119,7 +119,10 @@ public ISwapProvider ResolveProvider(SwapRoute route, string? preferredProviderI
     // Local signing sources pass aux_rand=null to BIP-340, so signatures are deterministic.
     // Remote-signer transports MUST honour the same convention or the preimage will rotate
     // per call and recovery will silently fail — see IRemoteSignerTransport.SignAsync.
-    private const string PreimageTag = "NArk-Boltz-Preimage-v1";
+    // Tag is protocol+provider scoped (Arkade brand, Boltz provider), not SDK-scoped, so any
+    // Arkade SDK implementing the same scheme produces the same preimage and can recover
+    // swaps the .NET SDK created (and vice versa). Versioned ("-v1") for future scheme evolution.
+    private const string PreimageTag = "Arkade-Boltz-Preimage-v1";
     private static readonly byte[] PreimageTagBytes = Encoding.UTF8.GetBytes(PreimageTag);
 
     private async Task<byte[]> DerivePreimageAsync(

docs/articles/swaps.md

@@ -110,14 +110,14 @@ Typical states recorded against each `ArkSwap`:
 The preimages we generate for reverse and chain swaps are derived deterministically from the wallet's signing material so that a restored wallet can re-derive them and claim outstanding VHTLCs. The scheme:
 
 ```
-message  = SHA-256( "NArk-Boltz-Preimage-v1" || descriptor.ToString() || u32_le(index) )
+message  = SHA-256( "Arkade-Boltz-Preimage-v1" || descriptor.ToString() || u32_le(index) )
 sig      = BIP-340-Schnorr( descriptor_key, message, aux_rand=null )
 preimage = SHA-256( sig )
 ```
 
 The signed input bundles three things:
 
-- **Tag** — domain-separates this signature from any other use of the descriptor's key. Versioned (`-v1`) so a future scheme bump can ship as `-v2` while recovery still tries v1 for older swaps.
+- **Tag** — domain-separates this signature from any other use of the descriptor's key. The string is intentionally protocol+provider scoped (`Arkade`+`Boltz`) rather than SDK-scoped, so an Arkade wallet implemented on top of any Arkade SDK (TypeScript, Go, Rust, .NET) can produce the same preimage from the same wallet material and recover swaps the .NET SDK created. Versioned (`-v1`) so a future scheme bump can ship as `-v2` while recovery still tries v1 for older swaps.
 - **Descriptor** — scopes the preimage to the specific swap descriptor.
 - **Index** — lets a caller derive multiple preimages from the same descriptor. Always `0` today; baked into v1 so recovery iteration is forward-compatible without a scheme bump.