Upgrade to nostr-sdk 0.43 (from 0.35)

Major upgrade of nostr-sdk dependency from 0.35 to 0.43 (8 minor versions).
All breaking API changes fixed, all tests passing.

Breaking Changes Fixed:
- EventBuilder::new() - Removed tags parameter, use .tags() method
- EventBuilder::to_event() → sign_with_keys() - Renamed signing method
- Client::new() - Takes ownership of keys (clone instead of reference)
- Relay::is_connected() - No longer async
- Client::get_events_of() → fetch_events() - Complete API redesign
- EventSource - Removed entirely
- Filter::custom_tag() - Takes single value instead of array
- Client::send_event() - Takes reference instead of ownership
- Multiple filters - Loop and combine instead of vec parameter
- Events type - New return type, convert with .into_iter().collect()

Files Modified:
- Cargo.toml: nostr-sdk = "0.43"
- src/audit.rs: EventBuilder API changes
- src/client.rs: Client, query, and filter API changes
- src/specs/nip01_smoke.rs: Event building changes

Documentation:
- NOSTR_SDK_0.43_UPGRADE.md: Comprehensive upgrade guide
- COMPILATION_FIXES.md: Marked as obsolete (0.35 fixes)
- SESSION_2025_11_04_SUMMARY.md: Session summary
- NEXT_SESSION_QUICKSTART.md: Updated status

Test Results:
✅ All 12 unit tests passing
✅ CLI builds successfully
✅ Examples build successfully
✅ Clean build with no warnings

Benefits:
- Latest stable nostr-sdk version
- Cleaner, more intuitive APIs
- Better performance (reference passing, sync operations)
- 8 versions of bug fixes and improvements
- Future compatibility

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