docs: new codeplain output layout — plain_modules/<module>/{code,tests}

[zanjonke]

codeplain now writes everything it generates under the module directory:

	old	new
implementation + unit tests	plain_modules/<module>/	plain_modules/<module>/code/
conformance tests	conformance_tests/<module>/<spec>/	plain_modules/<module>/tests/<spec>/

Verified against the installed CLI rather than assumed — codeplain --help documents the unit-test script as receiving "the module's code folder path (e.g. plain_modules/module_name/code)" and the conformance script as receiving "plain_modules/module_name/code" plus "a subfolder of the module's tests folder (e.g. plain_modules/module_name/tests/subfoldername)".

What changed

Rules — impl-reqs (unit tests sit in code/), test-reqs (conformance tests live in tests/, one folder per spec), requires-modules (the copied starting point is <required_module>/code), integration-embedded (+ directory tree), integration-embedded-testing (script arg contract and the ***test reqs*** example).

References — project-model.md repository tree, rendering-and-testing.md read-only surface, script roles and invocation shapes.

Skills — load-codeplain-reference, debug-specs, run-codeplain, create-requires-module, forge-plain, init-config-file, and the three implement-*-script skills.

Docs — CLAUDE.md, AGENTS.md, README.md.

Two things worth a second look

• conformance-tests-folder removed from init-config-file. That option no longer exists in the installed CLI (--copy-conformance-tests now reads from <build-folder>/<module>/tests), so documenting it would send users to a key the renderer rejects. conformance-tests-dest survives and is reworded.
• .memory/ left at the module root. run-codeplain documents renderer memory at plain_modules/<module>/.memory/. I couldn't confirm whether that moved under code/ — no new-layout render was available to check against. Worth verifying on the next render.

Untouched on purpose: the assets/ test scripts take their folders as arguments, so they're layout-agnostic, and run_conformance_tests_<lang> remains the script filename everywhere.