implement CEP-35 stateless session discovery and capability learning

[harsh04044]

CEP-35 has been drafted (contextvm-docs PR ContextVM/contextvm-docs#39) and standardizes the runtime discovery exchange already present in the TS SDK.

Right now the rs-sdk can send and receive encrypted MCP messages and publish static announcements, but it never attaches capability tags to outbound messages, never reads capability tags from inbound messages, and never adapts its behavior based on what the peer supports.

This issue tracks porting the TS SDK's capability learning and discovery tag exchange behavior to the Rust SDK, following the CEP-35 spec as the implementation guide.

1. Discovery tags module (foundation)

• get_discovery_tags(tags): filters out routing tags (p, e), preserves everything else (feat: add discovery tags module and server config foundation for CEP-35 #51)
• learn_peer_capabilities(tags) -> PeerCapabilities: checks for support_encryption, support_encryption_ephemeral, support_oversized_transfer (feat: add discovery tags module and server config foundation for CEP-35 #51)
• parse_discovered_peer_capabilities(tags): uses get_discovery_tags internally (feat: add discovery tags module and server config foundation for CEP-35 #51)
• merge_discovery_tags(existing, new): removed from TS SDK per CEP-35 spec update. Later tags must NOT merge into session baseline.
• Add SUPPORT_OVERSIZED_TRANSFER to src/core/constants.rs (feat: add discovery tags module and server config foundation for CEP-35 #51)
• Add gift_wrap_mode: GiftWrapMode to NostrServerTransportConfig (feat: add discovery tags module and server config foundation for CEP-35 #51)
• Add wrap_kind: Option<u16> to RouteEntry in src/transport/server/correlation_store.rs (feat: add discovery tags module and server config foundation for CEP-35 #51)

2. Session store enhancement

• Add has_sent_common_tags: bool to ClientSession (feat: add tag composition, server discovery tag emission and capability learning for CEP-35 #57 )
• Add supports_encryption: bool to ClientSession (feat: add tag composition, server discovery tag emission and capability learning for CEP-35 #57 )
• Add supports_ephemeral_encryption: bool to ClientSession (feat: add tag composition, server discovery tag emission and capability learning for CEP-35 #57 )
• Add supports_oversized_transfer: bool to ClientSession (feat: add tag composition, server discovery tag emission and capability learning for CEP-35 #57 )
• Replace unbounded HashMap with LRU-bounded session store (max_sessions default 1000)

3. Tag composition architecture

• Add compose_outbound_tags(base, discovery, negotiation) to BaseTransport. Canonical tag ordering: routing, then discovery, then negotiation. (feat: add tag composition, server discovery tag emission and capability learning for CEP-35 #57 )

4. Client-side discovery tag emission

• Add get_client_capability_tags(): constructs tags based on encryption mode and gift wrap mode
• Add get_pending_client_discovery_tags() with one-shot flag (has_sent_discovery_tags)
• Add build_outbound_client_tags(): composes routing + discovery tags
• Wire into send(): attach discovery tags on first request, flip flag after send

5. Server-side discovery tag emission

• Add inline get_common_tags() helper: builds server info + capability tags from config (feat: add tag composition, server discovery tag emission and capability learning for CEP-35 #57 )
• Add take_pending_server_discovery_tags(session): returns common tags on first response per client, empty after (feat: add tag composition, server discovery tag emission and capability learning for CEP-35 #57 )
• Add build_server_outbound_tags(): composes routing + discovery + negotiation tags (feat: add tag composition, server discovery tag emission and capability learning for CEP-35 #57 )
• Wire into send_response(): include discovery tags on first response to each client (feat: add tag composition, server discovery tag emission and capability learning for CEP-35 #57 )
• Wire into send_notification() -- include discovery tags on first notification to each client (feat: add tag composition, server discovery tag emission and capability learning for CEP-35 #57)

6. Client-side capability learning

• Add discovered_server_capabilities: PeerCapabilities to NostrClientTransport
• Add server_initialize_event: Option<Event>: stores first event carrying discovery tags
• Add learn_server_discovery(event): parses tags, OR-assigns capabilities, stores baseline event
• Wire into client event loop: call after decryption, before correlation routing
• Add public accessors: get_server_initialize_event(), discovered_server_capabilities()

7. Server-side capability learning

• On each inbound event, call learn_peer_capabilities() on inner event tags (feat: add tag composition, server discovery tag emission and capability learning for CEP-35 #57 )
• OR-assign learned capabilities to ClientSession fields (feat: add tag composition, server discovery tag emission and capability learning for CEP-35 #57 )

8. Announcement Manager refactor (deferred, last)

• Extract inline get_common_tags() into dedicated AnnouncementManager struct (refactor: extract AnnouncementManager from NostrServerTransport (CEP-35) #74 )
• Add get_server_info_tags(), get_capability_tags(), get_common_tags() with caching (refactor: extract AnnouncementManager from NostrServerTransport (CEP-35) #74 )
• Add set_extra_common_tags(), set_internal_common_tags(), set_pricing_tags() (refactor: extract AnnouncementManager from NostrServerTransport (CEP-35) #74 )
• Refactor announce(), publish_tools() etc. to delegate to AnnouncementManager (refactor: extract AnnouncementManager from NostrServerTransport (CEP-35) #74 )

Reference

• CEP-35 spec: docs(cep): add CEP-35 stateless session discovery specification contextvm-docs#39
• CEP-35 spec update (baseline vs local event capabilities): ContextVM/sdk@523a474
• TS SDK: src/transport/discovery-tags.ts
• TS SDK: src/transport/nostr-client-transport.ts:227-281, 853-909
• TS SDK: src/transport/nostr-server-transport.ts:441-469, 985-992