[thirtytwobits]

Makes the full CETLVaSt suite build and pass with GCC 7 (7.5.0), and adds GCC 7 to CI. The Arduino ecosystem is still on GCC 7, so this is a step toward embedded/Arduino support. Refs #149, refs #136.

Changes

Library

• type_traits_ext.hpp (is_convertible_without_narrowing static assert is failing in some cases with GCC v7.3.1 #136): is_convertible_without_narrowing is now spelled as copy-list-initialization of a by-value function parameter. GCC 7 rejects the old std::array<To, 1>{{{from}}} for scalar To. GCC 7 doesn't diagnose narrowing in constructor arguments during list-initialization of a class type, so for that case it now behaves like std::variant. The trait docs and test_type_traits_ext.cpp describe this. This commit (0ad3b87) is the fix that was sitting uncommitted on the issue/136 working tree. If is_convertible_without_narrowing static assert is failing in some cases with GCC v7.3.1 #136 lands separately, drop it.
• pf20/span.hpp: the CDE_span_002 debug assert now casts the iterator distance to size_type explicitly before comparing it with Extent. The comparison already did this conversion implicitly, so behaviour is the same on every compiler. GCC 7 flags the implicit version with -Wsign-conversion (Debug builds only).

Tests only

• test_variable_length_array_exception_safety.cpp: GCC 7 deduces const auto list = {Value{...}, ...} as std::initializer_list<const Value> (GCC PR c++/63149, fixed in GCC 8), so no constructor matched. VariableLengthArray itself is correct. The fix is to spell out std::initializer_list<Value>, both in the constructor test and in the initializer-list assignment test that Complete VLA range and initializer-list assignment #238 added.
• pmr/test_pmr_function.cpp: clear functions with = nullptr instead of = {}. Both resolve to operator=(std::nullptr_t), but GCC 7 reports the {} form under -Wzero-as-null-pointer-constant.

CI

• .github/workflows/cetlvast.yml: adds a gcc7-native / cpp-14 entry to the arm64 verification matrix through include:, so GCC 7 isn't combined with cpp-17/20. It uploads the same artifacts as the other entries. The gcc7-native presets only exist for cpp-14.

Verification

Rebased onto main after #238 and re-verified:

• cmake --workflow --preset workflow-gcc7-native-cpp-14-offline passes end to end in ghcr.io/opencyphal/toolshed:ts24.4.3 (aarch64) from a clean build dir. All 5 steps pass, including run_unittests and run_examples in Release, ReleaseEP, Debug and DebugEP: 216 passing suite runs.
• Host workflow-gcc-native-cpp-14-offline (Homebrew GCC 13.4, macOS) passes end to end, also 216 passing suite runs. Clang and the other standards are left to CI.
• Changed lines pass the toolshed's clang-format (clang-format-diff with clang-format 19.1.7).

🤖 Generated with Claude Code

[MAMONTBOT]

Reviewed the changes; no blocking issues found.