Add option for explicit UART socket

[kurtjd]

Previously this would always have the EC produce a PTY. However, needed flexibility to instead pass in an explicit path to make it easier to use this with the host QEMU over in the qemu arm virt repo. So we just add that option, but keep the default PTY behavior if the env variable isn't set.

Resolves #43

[Copilot]

🟡 Changes recommended

The new rm -f "$EC_UART_SOCK" cleanup is unsafe without -- and a type check, since it can be option-injected and can delete non-socket files at an arbitrary path.

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Pull request overview

Adds an opt-in environment variable to let dev-qemu use a stable UNIX-domain socket for UART instead of always allocating a PTY, improving interoperability with external QEMU co-simulation setups (per Issue #43) while preserving the existing default behavior.

Changes:

• Document EC_UART_SOCK in dev-qemu README and configuration table.
• Extend qemu-ec.sh to select between -serial pty (default) and a socket-backed -chardev ... -serial chardev:... when EC_UART_SOCK is set.
• Preserve the “char device redirected to … (label serial0)” announcement behavior for consumers that grep it (notably the headless path).

File summaries

File	Description
platform/dev-qemu/README.md	Documents the new UART socket option and how to enable it via EC_UART_SOCK.
platform/dev-qemu/qemu-ec.sh	Implements runtime selection between PTY UART and a stable UART socket based on EC_UART_SOCK.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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