refactor(transport): break down oversized transport modules after PR #35

[1amKhush]

Summary

PR #35 (Feat/oversized payloads) successfully landed CEP-22 oversized transfer support, but it grew nostr-client-transport.ts and nostr-server-transport.ts well beyond the recommended LOC number:

File	LOC	Status
nostr-server-transport.ts	1,148	⚠️ ~1.6x over guideline
nostr-client-transport.ts	1,139	⚠️ ~1.6x over guideline

Several self-contained concerns can be extracted into focused submodules without changing public API or behavior:

• Shared tag utilities — hasSingleTag, hasEventTag are duplicated across both transports.
• Client-side oversized sending — sendOversizedRequest + sendFrame closure (~95 LOC) is a standalone state machine.
• Client relay resolution — resolveOperationalRelayHandler + connectFallbackOperationalRelays (~100 LOC) is self-contained relay discovery/fallback logic.
• Server-side oversized handling — sendOversizedResponse + sendAcceptFrame (~65 LOC) are independent frame publishing helpers.
• Capability learning — inline hasSingleTag(event.tags, ...) capability checks can be consolidated into a reusable, extensible function.

Non-Goals

• No behavioral changes , this is a pure refactor.