aarch64 BYE crash: add heap-address diagnostics, rule out gdb debugging on this target

Continued investigating the aarch64 BYE cold-restart exception (FABRIC-2.md
Section I). Added permanent boot diagnostics: kmalloc_heap_base_addr()/
kmalloc_heap_end_addr() now print in print_heap_stats(), confirming the
fault address is provably inside the kmalloc heap (not kernel code, not
firmware). Bumped aarch64 QEMU RAM to 4096MB to test heap-placement
sensitivity (no effect -- heap size is a fixed 2GiB default, independent
of total RAM once "enough" exists).

Three separate live gdb debugging attempts (software breakpoint, hardware
breakpoint on arch_cold_reset, hardware breakpoint on mama_word_bye's
entry) all silently failed to fire despite disassembly-confirmed-correct
addresses and confirmed execution reaching those points. A sanity check
(hbreak on console_println, called thousands of times per boot) also never
fired even 8802 lines into a serial log -- conclusively a gdbstub/QEMU
tooling limitation for this aarch64 target, not a kernel-side finding.
Live single-stepping is not currently viable here; documented so it isn't
re-attempted the same way.

Root cause still open. Full trail in FABRIC-2.md Section I.

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

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