refactor: split SDI serial watch helpers

[dso2ng]

Summary

• Split the SDI serial monitor path into smaller helpers for port discovery, output sinking, and timestamped printing.
• Keep the existing watch_serial() public behavior and default WCH-Link CDC auto-detection unchanged.
• Add unit tests for WCH-Link CDC VID/PID matching and timestamped line formatting across chunks.

Motivation

This makes the SDI serial watch path easier to extend in later changes, such as explicit port/probe selection or alternate output formats, while keeping this PR limited to an internal refactor.

Test Plan

• cargo test
• cargo build