Sparrow Wallet support

[msgilligan]

This is a meta-issue for Sparrow Wallet support.

@craigraw identified the following issues (originally reported in Issue #79) when creating a prototype build of Sparrow Wallet using secp256k1-jdk.

Resolved

• Issue API: add simple implementations of P256K1XOnlyPubKey and P256k1PrivKey #139

Scheduled for 0.2

• Issue Grind ECDSA signatures for low-R (RFC6979) #136
• Some follow-up API improvements related to Issue 139

Scheduled for 0.3

• Issue Add the secp256k1_ecdh function to the secp-api and FFM implementation. #138

Unscheduled

• Issue FFM: Ability to load libsecp256k1 from a JAR #137

[craigraw]

LGTM

[msgilligan]

Sparrow found a bug in the JDK (in jpackage) that is currently preventing Sparrow from upgrading to JDK 25. Hopefully the fix will be in JDK 25.0.1.

Here are some reference/tracking links:

• Issue: https://bugs.openjdk.org/browse/JDK-8345810
• (Integrated) PR for JDK 26: 8345810: Custom launchers must be linked with pthread to avoid dynamic linker issues openjdk/jdk#26978
• backport PR for JDK 25: 8345810: Custom launchers must be linked with pthread to avoid dynamic linker issues openjdk/jdk25u#257