refactor(test): split codegen regressions into focused suites

tests/codegen/README.md

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+# Code generation regression suites
+
+`tests/codegen_regressions.rs` remains the Cargo integration-test target so existing
+CI and release commands using `--test codegen_regressions` and a test-name filter
+keep selecting the same cases.
+
+The tests are grouped by purpose in `language.rs`, `abi.rs`, `platform.rs`
+and `diagnostics.rs`. Shared setup, target gates, temporary fixture isolation,
+CLI helpers and timeouts live in `support.rs`.
+
+Run every regression:
+
+```sh
+cargo test --locked --test codegen_regressions
+```
+
+Run an individual regression:
+
+```sh
+cargo test --locked --test codegen_regressions riscv64_c_abi_interoperates_with_c
+```