x86/multikernel: inherit host timer calibration - #5

Summary

Spawned x86 kernels cannot safely repeat timer calibration while the primary kernel owns the legacy PIT, PIC, and I/O APIC resources used by those probes.

This commit appends optional calibration fields to the spawn context and records the primary kernel's delay-loop, CPU, TSC, and local APIC timer measurements when preparing a spawned kernel. The spawned kernel reuses those measurements when explicit command-line overrides are absent.

Existing context offsets remain unchanged. The appended fields are zero-initialized, so unavailable measurements retain the existing fallback behavior, while explicit overrides remain authoritative.

Validation

• strict checkpatch with no errors, warnings, or checks
• focused platform-quirks.o and Multikernel spawn object builds
• full CONFIG_MULTIKERNEL=y kernel build
• full CONFIG_MULTIKERNEL=n vmlinux build
• source and model checks for offset preservation, zero-value fallback, explicit overrides, and inherited calibration
• two sequential QEMU integration passes, each completing all 83 required markers