feat: telemetry foundation (TCP client + framing + Node Info)

[HanaYukii]

Sub-issue of #775 (JIP-3 Telemetry).

Scope

Foundation for JIP-3 telemetry. No emission yet, only the wire/runtime substrate that all subsequent stages build on.

• TCP client with 4-byte LE length framing
• Node Info encoding (no timestamp, no discriminator; first byte is protocol version)
• Event ID counter: per-connection, post-increment from 0
• Multi-range drop window (dropRange{first_id, count, first_ts, last_ts}) with sequencer mutex around ID + enqueue/drop record
• Reconnect with exponential backoff (1s → 30s, infinite retry)
• --telemetry HOST:PORT CLI flag
• Client API: Enabled() / Emit / EmitLazy(disc, builder) / EmitFollowup / EmitFollowupLazy / Close
• InvalidID = ^uint64(0) sentinel for disabled / reconnecting / degraded states
• Writer goroutine with defer recover() and graceful Close (drain envelopes, flush drop_ranges, FIN, 5s timeout)
• Common-type codec: Option<T>, String<N> (UTF-8 + len cap), len++[T], bool, fixed-int LE, Reason
• Contract test framework skeleton at internal/telemetry/contract_test.go
• Golden vector fixtures at testdata/wire_fixtures/

Dependencies

• Parent: JIP-3: Telemetry #775
• Q-blocker: none (Q1-independent; foundation can land before main loop work resolves)
• Design: docs/jip3-telemetry-design.md (Architecture, Wire Protocol, Event ID + Lazy API + Backpressure)

Acceptance

• All codec roundtrip + golden vector tests pass
• mockTcpServer integration: lifecycle, reconnect, concurrent emit, interleaved drop, bridge invariants, panic recover, Close
• E2E: docker-compose JamTART, Node Info on /api/ws, stress overflow validates Dropped ID alignment
• Reviewer signoff