feat(platform-wallet): add gap-limit identity discovery scan

[QuantumExplorer]

Issue being fixed or feature implemented

Implements DashSync-style gap-limit identity discovery for the platform-wallet crate, enabling wallet sync to find registered identities by scanning DIP-13 authentication key hashes against Platform.

What was done?

New: Identity Discovery Scanner

Added discover_identities() and discover_identities_with_contacts() methods to PlatformWalletInfo that:

1. Derive ECDSA authentication keys at consecutive DIP-13 indices (m/9'/COIN_TYPE'/5'/0'/0'/identity_index'/0')
2. Hash each public key with RIPEMD160(SHA256) to produce a 20-byte hash
3. Query Platform via Identity::fetch(&sdk, PublicKeyHash(hash))
4. Add discovered identities to the IdentityManager
5. Stop scanning after gap_limit (typically 5) consecutive misses
6. Optionally fetch DashPay contact requests for each discovered identity

This mirrors the pattern used by DashSync iOS (DSIdentitiesManager.retrieveIdentitiesByKeysWithCompletion:) which derives 5 consecutive keys beyond the last known used identity index and batch-queries Platform.

Refactored: Shared Utilities

• Extracted key derivation logic into key_derivation.rs (derive_identity_auth_key_hash)
• Moved parse_contact_request_document to shared location in mod.rs
• matured_transactions.rs now uses these shared helpers (no API changes)

How Has This Been Tested?

• cargo check -p platform-wallet passes cleanly
• Existing tests pass unchanged

Breaking Changes

None. All existing public APIs are unchanged.

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional tests
• I have made corresponding changes to the documentation

For future work:

• Expose via FFI for Swift SDK integration
• Wire into SPV sync completion event
• Add unit tests with mock SDK

🤖 Generated with Claude Code

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[github-actions]

✅ DashSDKFFI.xcframework built for this PR.

• Workflow run: https://github.com/dashpay/platform/actions/runs/22714248634
• Artifacts: DashSDKFFI.xcframework (folder), DashSDKFFI.xcframework.zip, xc_checksum.txt

SwiftPM (host the zip at a stable URL, then use):

.binaryTarget(
  name: "DashSDKFFI",
  url: "https://your.cdn.example/DashSDKFFI.xcframework.zip",
  checksum: "35f483c483c97d2313c5429ca821e18553841a6defefc3cc9acbcd2a95eb115a"
)

Xcode manual integration:

• Download 'DashSDKFFI.xcframework' artifact from the run link above.
• Drag it into your app target (Frameworks, Libraries & Embedded Content) and set Embed & Sign.
• If using the Swift wrapper package, point its binaryTarget to the xcframework location or add the package and place the xcframework at the expected path.

[thepastaclaw]

@coderabbitai review

[thepastaclaw]

Follow-up refactor in #3206 — extracts duplicated contact request logic, replaces eprintln with tracing, names magic constants, and logs previously-swallowed parse errors.