test: add big-endian architecture Miri validation

[membphis]

Summary

Add powerpc64-unknown-linux-gnu (big-endian) target to the Miri test matrix, aligning with serde_json's QA practices for endianness validation.

Changes

• CI: Add powerpc64 Miri FFI tests (ffi_smoke, ffi_cursor, ffi_strings, ffi_ops_interleave) using scalar scanner only (AVX2 is x86_64-only)
• Tests: Fix c_char signedness for cross-platform compatibility — char is unsigned on PowerPC, signed on x86. Changed as *const i8 to as *const c_char in 6 test files
• Docs: Add ENDIAN NOTE to src/scan/avx2.rs documenting that the AVX2 scanner is x86_64-only and relies on ISA-fixed bit semantics rather than memory byte order

Why the c_char fix matters

The big-endian Miri test immediately caught a latent ABI mismatch: test files were casting byte pointers to *const i8, but FFI functions expect *const c_char. On PowerPC (and ARM in some configurations), c_char is u8, not i8. This would have broken FFI calls on those platforms.

This demonstrates exactly why big-endian testing is valuable — it surfaces cross-platform assumptions that don't manifest on x86_64.

Verification

• All powerpc64 Miri tests pass locally
• All x86_64 tests pass (cargo test --release)
• Clippy clean (make lint)

Closes #148

Summary by CodeRabbit

• Tests

  • Continuous integration now validates FFI functionality and library compatibility across additional processor architectures, including big-endian systems (PowerPC64), beyond existing x86 testing
  • Updated FFI pointer types consistently throughout test suites for improved type correctness and cross-platform compatibility

• Documentation

  • Added documentation clarifying endianness behavior and assumptions in performance-optimized code components

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: d14c2822-1153-486b-a0cb-7342ebf30dfa

📥 Commits

Reviewing files that changed from the base of the PR and between 4ad9d98 and 816cc2f.

📒 Files selected for processing (8)

• .github/workflows/ci.yml
• src/scan/avx2.rs
• tests/ffi_cursor.rs
• tests/ffi_numbers.rs
• tests/ffi_strings.rs
• tests/ffi_typeof.rs
• tests/ffi_wide_object.rs
• tests/third_party_fixtures.rs

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📝 Walkthrough

Walkthrough

This PR implements big-endian (PowerPC64) architecture validation in the Miri CI test matrix and improves FFI type safety across the test suite by using c_char instead of raw i8 for pointer casts in all FFI function calls.

Changes

Big-Endian Testing Infrastructure and FFI Type Correctness

Layer / File(s)	Summary
CI infrastructure for big-endian Miri testing .github/workflows/ci.yml	CI sanitizers job adds powerpc64-unknown-linux-gnu target to nightly Rust setup and introduces a new Miri FFI test step that validates the scalar scanner against big-endian architecture using the same FFI test suite (ffi_smoke, ffi_cursor, ffi_strings, ffi_ops_interleave).
AVX2 endianness assumptions documentation src/scan/avx2.rs	Module-level comments document that the AVX2 scanner is x86_64-only (little-endian), explain how _mm256_movemask_epi8 bit-to-lane mapping is fixed by ISA rather than memory byte order, and note that big-endian CI coverage runs only the scalar scanner.
FFI test imports: add c_char type tests/ffi_cursor.rs, tests/ffi_numbers.rs, tests/ffi_typeof.rs, tests/ffi_strings.rs, tests/ffi_wide_object.rs, tests/third_party_fixtures.rs	All test files update std::os::raw imports to include c_char, providing the correct FFI character type for subsequent pointer casts.
Cursor and string getter FFI type updates tests/ffi_cursor.rs	qjson_open, cursor field/string/numeric getters, and container operation calls are updated to cast key and path pointers as *const c_char instead of *const i8 across nine test functions.
Number getter FFI type updates tests/ffi_numbers.rs	All calls to qjson_get_i64, qjson_get_u64, qjson_get_f64, qjson_get_bool, qjson_open, and qjson_cursor_get_u64 are updated to cast key/path pointers as *const c_char across 12 test functions.
String getter FFI type updates tests/ffi_strings.rs	Three qjson_get_str call sites are updated to cast path.as_ptr() as *const c_char instead of *const i8.
Type inspection FFI type updates tests/ffi_typeof.rs	Fourteen test functions update qjson_typeof, qjson_is_null, qjson_len, qjson_get_f64, and qjson_get_bool calls to cast name/path pointers as *const c_char.
Wide object sampling FFI type updates tests/ffi_wide_object.rs	Forward, reverse, and unknown-key sampling loops in qjson_get_i64 calls are updated to cast key pointers as *const c_char.
Third-party fixtures FFI type updates tests/third_party_fixtures.rs	FFI wrapper functions (get_str, get_i64, get_bool, is_null, len) and menu/matrix/invalid-path fixture tests are updated to cast path/key/empty pointers as *const c_char in all qjson API calls.

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 6

✅ Passed checks (6 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: adding big-endian architecture Miri validation to the test matrix.
Linked Issues check	✅ Passed	All functional requirements from issue #148 are met: powerpc64 target added to CI, scalar scanner tests enabled, AVX2 documented, and c_char casting fixed for cross-platform compatibility.
Out of Scope Changes check	✅ Passed	All changes align directly with requirements from issue #148: CI configuration, test type casting fixes for FFI ABI compatibility, and documentation of endianness assumptions.
E2e Test Quality Review	✅ Passed	PR adds CI infrastructure and type fixes to existing tests, not new E2E tests. The custom check for E2E test quality is not applicable to this infrastructure/compatibility-focused work.
Security Check	✅ Passed	PR adds big-endian Miri validation to JSON parser library with proper c_char type safety, endian documentation, and CI workflow updates; no violations of 7 security categories found.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch test/big-endian-miri-validation

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