EPIC 1669 SI-35: Consolidate duplicate UDP types

[josecelano]

Parent EPIC: #1669 — Overhaul: Packages
EPIC spec: docs/issues/open/1669-overhaul-packages/EPIC.md

Goal

Eliminate duplicate type definitions and constants in the UDP tracker packages by consolidating them into their canonical locations.

Background

A workspace-wide audit of UDP-related packages found that the UDP layer is significantly cleaner than the HTTP layer — the core protocol types are defined exclusively in packages/udp-protocol/src/ and imported everywhere else. This is the correct architecture.

However, three duplications were found:

🔴 ConnectionContext — full copy-paste

The struct and its entire impl block are duplicated between packages/udp-core/src/event.rs and packages/udp-server/src/event.rs. The only difference is field visibility (pub in core, private in server). The impl blocks are identical.

🟡 MAX_PACKET_SIZE — same constant, two locations

Both packages/udp-server/src/lib.rs and packages/tracker-client/src/udp/mod.rs define pub const MAX_PACKET_SIZE: usize = 1496;. The tracker-client already depends on udp-protocol; this constant could live there and be shared.

🟡 PROTOCOL_ID — dead code copy

packages/tracker-client/src/udp/mod.rs defines pub const PROTOCOL_ID which is unused — zero references anywhere. It mirrors PROTOCOL_IDENTIFIER from udp-protocol. Should be removed.

🟢 Intentional duplications (not in scope)

AnnounceEvent, InfoHash, and NumberOfBytes are kept separate per ADR 20260527175600. As part of this issue, each location will gain an adr: comment so future contributors understand the architectural reasoning and do not accidentally re-couple the types.

Scope

In Scope

• Consolidate ConnectionContext into a single canonical definition (likely in udp-core)
• Move MAX_PACKET_SIZE to udp-protocol and import it in both udp-server and tracker-client
• Remove the unused PROTOCOL_ID constant from tracker-client
• Add adr: comments to intentional-duplication locations referencing the ADR
• Verify all tests pass and no functionality regresses

Out of Scope

• Merging protocol-level types (AnnounceEvent, InfoHash, NumberOfBytes) — governed by ADR
• Changing the public API of udp-protocol beyond what's needed for consolidation
• Refactoring the UDP server architecture

References

• Spec: docs/issues/drafts/1669-si-35-consolidate-duplicate-udp-types.md
• Parent EPIC: #1669
• Decisions log: docs/issues/open/1669-overhaul-packages/DECISIONS.md
• Related ADR: docs/adrs/20260527175600_keep_protocol_and_domain_types_decoupled.md