[Task] Refactor TCP/UDP to per-operation completion context for concurrency safety

[crrow]

Goal

Refactor TCP/UDP operation state handling to support safe concurrent operations by using per-operation completion/context instead of single shared mutable completion state.

Scope

In scope:

• pkg/xev/tcp.go
• pkg/xev/udp.go
• pkg/cxev/tcp.go
• pkg/cxev/udp.go
• related tests

Out of scope:

• changing the overall package split (xev/cxev)
• adding new network protocol features

Problem Statement

TCPConn/UDPConn currently reuse single completion/callback fields across operations, which can cause callback ID overwrites and race-prone behavior under overlapping read/write/close sequences.

Definition of Done

• Introduce per-op state object for TCP/UDP similar to robust file-op pattern.
• Ensure read/write/close overlap is either safely supported or explicitly rejected with deterministic errors.
• Add tests for overlapping operations and callback correctness.
• just check passes.
• just test-quick passes.

Verification Commands

just check
just test-quick
go test ./pkg/xev -run 'TCP|UDP' -race -count=1

Performance and Latency Budget

• Throughput regression <= 3%.
• p99 latency regression <= 5%.

Worktree Branch Name

codex/issue-2-per-op-completion-tcp-udp

[crrow]

Leader dispatch: Wave 2 (P1). Run after #9/#11 to reduce overlap risk in lifecycle and concurrency refactors.