feat: expose serialized extended public keys

[ben-kaufman]

Summary

• add serialized_extended_pubkey using Rust Bitcoin's BIP32 Xpub parser and canonical 78-byte encoder
• reject invalid Base58 characters, invalid checksums, and Base58Check payloads that are not valid extended public keys
• expose the result as Swift Data, Kotlin ByteArray, and Python bytes
• rebase onto current master, including the latest Boltz and Android publishing changes
• bump the release version from 0.5.4 to 0.5.5 and regenerate all platform bindings
• use master's current iOS artifact policy: ship the XCFramework zip without tracking duplicate unpacked static libraries

Verification

• ./build.sh all — passed for iOS, all four Android ABIs, and Python
• Android Maven Local publish validation — passed
• cargo test extended_pubkey --locked — 6 passed
• generated APIs verified as Swift Data, Kotlin ByteArray, and Python bytes

The strict repository gates remain blocked by existing master issues: cargo fmt --check reports formatting drift in activity/backup_migration.rs, and cargo clippy -- -D warnings reports 30 pre-existing warnings across unrelated modules. The new extended-key module introduces none.