Feature/serial buffer options

[JPHutchins]

Long overdue feature to allow setting the "line length" and "line buffers". "mtu" is deprecated, but will continue being interpreted as it always has: line_length = mtu, line_buffers = 1.

Many investigations and conversations have confirmed that the line length never should have been configurable in the first place. Including in Zephyr, MCUboot etc. But, we need to continue supporting non-standard line lengths since there are many units in the field that changed the line length as a speed up.

Also committed is an LLM generated report regarding naming and defaults.

[Copilot]

Pull request overview

This PR adds explicit serial transport framing/buffering controls to the smpmgr CLI while keeping --mtu working for backwards compatibility (now deprecated for serial).

Changes:

• Introduces --line-length and --line-buffers CLI options and threads them into the runtime Options.
• Updates serial transport initialization to derive max_smp_encoded_frame_size = line_length * line_buffers, with legacy --mtu mapped to line_length=mtu, line_buffers=1 and guarded against mixed usage.
• Adds an LLM-generated research note on SMP serial parameter naming/defaults and updates VS Code workspace settings for Poetry env/package management.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
smpmgr/main.py	Adds CLI flags for serial line length/buffer count; updates --mtu help text and passes new values into Options.
smpmgr/common.py	Adds defaults and implements serial transport argument selection/validation (including --mtu deprecation behavior).
docs/llm/smp-serial-naming-research.md	Adds a research document supporting naming/default decisions for SMP serial parameters.
.vscode/settings.json	Sets Poetry as the default env/package manager for VS Code’s Python environment tooling.

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