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
• bump the release version from 0.4.1 to 0.4.2 and regenerate all platform bindings
• strip debug symbols from iOS release archives so generated XCFramework libraries stay below GitHub's file-size limit while preserving exported UniFFI symbols

Verification

• cargo test extended_pubkey --locked — 6 passed
• cargo clippy --locked — passed with existing repository warnings
• ./build.sh android, ./build.sh ios, and ./build.sh python — passed
• cargo test --locked — 442 passed, 25 ignored; 11 live Blocktank staging tests timed out

The strict cargo clippy -- -D warnings gate remains blocked by warnings already present on master; the new extended-key module introduces none.