aarch64: fix BYE cold-restart crash — PSCI SYSTEM_RESET via HVC, not SMC

Root cause of the aarch64 BYE cold-restart exception (present since at
least 2026-08-08, ESR_EL1=0x02000000/EC=0 "Unknown reason"), found via
live gdb single-stepping through the actual crash: arch_cold_reset()
issued PSCI SYSTEM_RESET via `smc #0`, but QEMU's aarch64 virt machine
booted with AAVMF (UEFI firmware, no genuine EL3/TrustZone secure
monitor) serves PSCI via HVC, not SMC -- nothing exists to answer an
SMC call, so it trapped as an illegal instruction straight into the
kernel's own exception handler. Not memory corruption, not a race --
a wrong conduit for this boot configuration.

Fix: smc #0 -> hvc #0. Function ID and calling convention unchanged.

Getting to this required first discovering that starkernel_kernel.elf
is not the binary that actually runs -- MONOLITHIC_BUILD links
kernel_main() directly into starkernel_loader.efi, a completely
separate, differently-linked build artifact. Every earlier gdb
breakpoint attempt this session failed because it used addresses from
the wrong file. Real addresses (UEFI-chosen ImageBase + linker-map
RVA) let gdb catch the crash live for the first time.

Verified: full aarch64 acceptance pass, 30/30 stress-campaign reps
PASS (unaffected -- this bug only manifested on BYE), and BYE now
exits cleanly with no exception for the first time in this
investigation.

Full writeup in FABRIC-2.md Section I.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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