Decouple kernel headers from glibc

[sighook]

Context

At present, Linux kernel headers are bundled inside the glibc and glibc-32 packages. This tight coupling makes maintenance harder and introduces conflicts when updating either glibc or kernel headers.

Goal

Split kernel headers into a dedicated linux-headers package.
This provides cleaner package separation and allows kernel headers to be managed independently, while ensuring glibc builds remain stable.

Transition Considerations

• Direct removal of headers from glibc would break builds due to file ownership conflicts.
• A staged approach is required:
  • Phase 1: introduce linux-headers with path separation (/usr/include/linux-headers-<version>/).
  • Phase 2: configure glibc to build against external headers.
  • Phase 3: temporarily duplicate headers in both packages.
  • Phase 4: drop headers from glibc once validated.

Child Issues

• Introduce linux-headers package (transition plan from glibc) #10: Transition plan for linux-headers package (implementation details).

Why This Parent Exists

This issue defines the overarching refactor theme.

• It explains the motivation.
• It links to child issues for concrete steps.
• It ensures contributors understand the architectural direction without diving into build scripts.

[sighook]

Once done:

• Ensure glibc and linux-headers remain in sync. Track glibc’s required kernel baseline (--enable-kernel) and update the linux-headers package accordingly, so builds remain reproducible and ABI‑safe. Pkgfiles should point to that.
• Fix the codebook's "Toolchain Update Order".