feat: client discovery tag emission and capability learning for CEP-35

[harsh04044]

Part of #43

Implements the client side of CEP-35, the client now advertises its capabilities on the first outbound request and learns server capabilities from inbound discovery tags.

Changes:

Client emits discovery tags (support_encryption, support_encryption_ephemeral) on the first request only. Tags are
built from the client's encryption and gift wrap mode config. One-shot per session, subsequent requests carry no discovery tags.

Client event loop now calls learn_server_discovery() on every inbound event after decryption. The first event carrying discovery tags sets the session baseline (frozen after that). Capabilities are OR-assigned and never downgraded.

Public accessors: get_server_initialize_event() and discovered_server_capabilities().

7 unit tests and 3 integration tests covering tag emission conditions, one-shot behavior, OR-assign semantics, baseline freeze.