[Task] Harden error messages and address parsing for diagnosability

[crrow]

Goal

Improve error quality and input validation for network/address and low-level error paths to make failures diagnosable and user-friendly.

Scope

In scope:

• pkg/xev/tcp.go (address parsing)
• pkg/cxev/tcp.go (TCPError formatting)
• pkg/cxev/udp.go (UDPError formatting)
• pkg/cxev/file.go (FileError formatting)
• README/API docs examples impacted by behavior changes
• tests

Out of scope:

• introducing a full custom error type hierarchy for every package

Problem Statement

Current error rendering and parsing paths can yield low-fidelity diagnostics (e.g., rune-converted error codes, permissive port parsing). This slows debugging and weakens usability.

Definition of Done

• Error messages include actionable numeric/context details.
• Address parser rejects invalid/non-numeric/out-of-range ports.
• Add tests for representative invalid inputs and error formatting.
• Sync docs/examples with current API signatures where needed.
• just check passes.
• just test-quick passes.

Verification Commands

just check
just test-quick
go test ./pkg/xev ./pkg/cxev -run 'Parse|Error' -count=1

Worktree Branch Name

codex/issue-4-error-and-parse-hardening

[crrow]

Leader dispatch: Wave 1 (P1). Start after Wave 0 merges. Focus on diagnostics and strict input validation.