Add fuzz test infra

[prozacchiwawa]

Note

Low Risk
Changes are isolated to test/CI code and a feature-gated optional dependency (subprocess). Main risk is CI flakiness/environment assumptions when invoking the installed 0.4.3 run binary.

Overview
Adds a new GitHub Actions workflow (pre_043_regression.yml) that installs chialisp@0.4.3 and runs a regression test suite with the preserved_043 feature enabled.

Introduces preserved_043 (and optional subprocess dependency) plus a new feature-gated test (cl26_fuzz_compare) that generates random, scope-correct CL26 programs and asserts the current compiler and 0.4.3 compiler produce identical assembled hex output.

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[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	cargo/​subprocess@​1.1.0

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[socket-security]

All alerts resolved. Learn more about Socket for GitHub.

This PR previously contained dependency changes with security issues that have been resolved, removed, or ignored.

Ignoring alerts on:

• cargo/winapi@0.3.9
• cargo/winapi-i686-pc-windows-gnu@0.4.0
• cargo/winapi-x86_64-pc-windows-gnu@0.4.0

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[prozacchiwawa]

@SocketSecurity ignore cargo/winapi-i686-pc-windows-gnu@0.4.0

[prozacchiwawa]

@SocketSecurity ignore cargo/winapi-x86_64-pc-windows-gnu@0.4.0

[prozacchiwawa]

@SocketSecurity ignore cargo/winapi@0.3.9

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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The base branch was changed.