fix(iOS): gate prebuilt headers on Swift C++-interop importability

[chrfalch]

Summary

Adds a generator-time gate so a Swift-C++-interop-hostile C++ type can never silently ship in the prebuilt iOS core headers again.

The class this guards: the prebuilt core headers ship as real clang modules. A C++ value type reachable from a shipped module whose implicit copy constructor is declared but ill-formed on instantiation — e.g. an implicit-copy struct with a std::vector<move-only-T> member — compiles fine in C++ but hard-errors when a Swift target with -cxx-interoperability-mode=default imports the module and its generated bridging uses the type as a copyable value. This is invisible to C++ CI (plain C++ never instantiates the copy), so it only surfaced as a broken community-library nightly (react-native-unistyles, via TraceRecordingState / HostTracingProfile — fixed in #57605).

The gate: a fourth compile gate (stage 3d) in headers-verify.js's runCompileGates, compiling a Swift TU with -cxx-interoperability-mode=default that imports every module declared by the composed ReactNativeHeaders module map (parsed at runtime, not hardcoded), plus a probe that forces the ClangImporter copyability path for a small explicit list of value types (SWIFT_CXX_INTEROP_PROBE_TYPES). Because a bare import does not eagerly instantiate copy constructors on the current toolchain, the probe reproduces what a real interop consumer's generated code does: for each listed type it conditionally instantiates a copy, which fails exactly as the real consumer fails if the type regresses to an implicit copy. Adding future coverage is a one-line list edit.

One module (React_RCTAppDelegate) is excluded as non-importable under interop by design (ObjC bootstrap module with C++-only factory conformances); the inspector C++ graph it would reach is covered directly by the probe instead. The exclusion is documented in-source, and new exclusions are required to carry a justification.

Independent review confirmed the probe reproduces the real failure for the right reason (the dangerous shape reports is_copy_constructible_v == true, so the probe enters the branch and forces the ill-formed body instantiation — mirroring ClangImporter, not masking the bug), and that both offender types are covered.

⚠️ Land after #57605 — the gate runs against the composed artifacts, which are built from source; until the move-only type fixes are in, the gate correctly fails the prebuild on the unfixed types.

Changelog:

[INTERNAL] [ADDED] - Prebuild gate that fails composed iOS headers unusable from a Swift C++-interop consumer

Test Plan

Against a freshly composed Debug artifact (node scripts/ios-prebuild -c -f Debug):

• Green: node scripts/ios-prebuild/headers-verify.js --flavor Debug passes the new gate with both probe types (TraceRecordingState, HostTracingProfile) fixed in the artifact.
• Red, TraceRecordingState: restoring the unfixed TraceRecordingState.h into the composed artifact fails the gate with the exact __construct_at / implicit-copy diagnostic; restoring the fix → green.
• Red, HostTracingProfile (proves coverage isn't accidental): same protocol with HostTracingProfile unfixed (TraceRecordingState left fixed) → gate fails naming HostTracingProfile and its probe specialization; restore → green.
• node --check, prettier --check, flow focus-check clean. Gate is skipped by --skip-compile like the other compile gates.

🤖 Generated with Claude Code

[meta-codesync]

@cipolleschi has imported this pull request. If you are a Meta employee, you can view this in D113011048.