docs: add README for common, runner and web CSE machine packages

[Akshay-2007-1]

Summary

Adds a README.md for each of the three CSE machine packages, following the same structure as the data-display READMEs in #38.

• src/common/cse-machine/README.md — protocol overview, constants table, and field-by-field documentation for all shared types (CseSnapshot, CseSerializedValue, CseSerializedInstruction, CseSerializedEnvFrame, CseSerializedBinding)
• src/runner/cse-machine/README.md — CseMachinePlugin API reference and usage example showing how an evaluator serializes steps and calls sendSnapshots
• src/web/cse-machine/README.md — CseMachineHostPlugin API reference, usage example showing how to extend it, and local dev serving instructions

Test plan

• READMEs render correctly on GitHub
• Cross-package links resolve correctly
• No existing source files modified

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Latest commit: 0a33205

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[gemini-code-assist]

Code Review

This pull request introduces comprehensive README documentation for the common, runner, and web packages of the CSE Machine visualizer, detailing their features, installation, structure, and usage. The review feedback suggests updating the documentation tables for CseSerializedValue and CseSerializedInstruction to explicitly mark the metadata fields as optional (unknown | undefined) to maintain consistency with the TypeScript definitions.

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[Akshay-2007-1]

@AaravMalani I am merging this for now as its easy to fix if there are issues.
I have taken inspiration from the READMEs of the data vis plugin after reviewing your PR.