chore: modernize header-only code to C++17 and remove dead code

[henryiii]

🤖 AI text below 🤖

This is a cleanup PR for the standalone header-only/ C++ library (which already requires C++17). It is public API for downstream C++ users — no public signatures or behaviors are changed. Only files under header-only/ are touched.

Changes

header-only/layout-builder/awkward/utils.h — drop the hand-rolled is_integral_v/is_signed_v/is_same_v/void_t "until we switch to C++17" shims and use std::is_*_v / std::void_t.

header-only/layout-builder/awkward/LayoutBuilder.h

• Replace the ~350 lines of BufferNBytesFunctor/ToBuffersFunctor/ToBufferFunctor/ToCharBuffersFunctor/SetIdFunctor/ClearFunctor helper classes — triplicated nearly verbatim across Record, Tuple, and Union "for C++11 compatibility" — with generic lambdas passed to visit_at. ContentsFormFunctor is intentionally left untouched to avoid conflicting with an in-flight form-bug-fix PR.
• Remove dead state that is written but never read: Union::last_valid_index_, IndexedOption::last_valid_ (its only real use, in extend_valid, is now a local), Tuple::field_index_, and the unused/uninitialized Empty::id_. Also drop the unused index_sequence locals in Union::to_buffers/to_buffer/to_char_buffers.
• The BitMasked class region is intentionally left alone.

header-only/growable-buffer/awkward/GrowableBuffer.h

• Drop the const qualifier on the options_ member so that GrowableBuffer (and every LayoutBuilder that contains one) is move-assignable again, and explicitly = default the move-assignment operator (the existing user-declared move constructor otherwise suppresses it). The options() accessor stays const.
• Use std::make_unique instead of unique_ptr(new ...) and drop a redundant std::move of a temporary.
• Fix append() so panel growth compounds geometrically: new panels are sized from the current reserved capacity (ceil(reserved * resize)) instead of always ceil(initial * resize) — previously, with default options, every panel was the same initial-sized 1024 elements, producing N/1024 linked panels. extend() now copies each contiguous segment with a memcpy when the element type is trivially copyable (the PRIMITIVE here always is) instead of element-by-element. Lengths and contents are identical.

header-only/tests/test_1542-growable-buffer.cpp — add two focused tests for the behavior-affecting changes: multi-panel geometric growth via append(), and move assignment (which previously failed to compile because of the const member). extend() correctness across panel boundaries remains covered by the existing test_extend.

Testing

cmake -S header-only -B build -DBUILD_TESTS=ON && cmake --build build builds cleanly. The growable-buffer, builder-options, and layout-builder-is-valid test executables pass, including the new tests.

Note: test_1494-layout-builder has a pre-existing BitMasked::clear() heap-use-after-free (reproducible on main before this PR) that is being fixed separately; it is unrelated to these changes and lives in the untouched BitMasked region.

AI assistance

This PR was prepared with Claude Code as part of an automated review of the header-only library. Changes were verified by building and running the test suite.

Merge order

Two other in-flight PRs touch GrowableBuffer.h (panel-copy/concatenate-offset bug fix) and the BitMasked class, so textual conflicts are expected at merge time. This PR should be rebased and merged after henryiii/fix-bitmasked-growablebuffer and henryiii/fix-layoutbuilder-form.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.42%. Comparing base (c05b885) to head (dfcf4dd).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

[ianna]

@henryiii - Thanks! This looks great.