fix(kernel): update Linux to 6.12.96

[appcypher]

TL;DR

Update the bundled stable kernel from Linux 6.12.95 to 6.12.96 and bump libkrunfw to 5.6.1 without changing ABI version 5.

Description

• Advance the bundled Linux 6.12.y baseline by one stable patch release.
• Bump the full libkrunfw version from 5.6.0 to 5.6.1 while retaining ABI version 5.
• Refresh the fixed build timestamp so generated release artifacts remain reproducible.
• Carry the kernel freshness check and complete build matrix merged after the 5.6.0 version update into the next firmware release.

Test Plan

• scripts/check-kernel-version.sh confirms 6.12.96 is the latest Linux 6.12.y release
• git diff --check origin/krunfw..HEAD exits 0
• Required kernel CI builds Linux x86_64, Windows x86_64, SEV x86_64, TDX x86_64, Linux aarch64, and Linux riscv64 (runs in GitHub CI; the local Docker backend failed before compilation with an overlay storage I/O error and host volume exhaustion)
• Boot the generated kernels through each supported libkrun runtime (needs platform-specific runtime validation)