docs: capture serial console output from boot, shell, and test runs

docs/shell-serial.log

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+
+========================================
+  frostbyte kernel  x86_64  long mode
+  multiboot2 / grub / freestanding gcc
+========================================
+
+[boot] multiboot2 magic verified (36d76289)
+[gdt]  loaded flat 64-bit segments
+[idt]  256 vectors installed (32 exceptions, 16 irqs)
+[pic]  remapped to 0x20-0x2f, irq0 and irq1 unmasked
+[pmm]  130559 KiB usable, 32736 frames, 32463 free after kernel
+[heap] bump allocator at 0x0000000000111000, 8192 KiB
+[paging] mapped 0x0000000040000000 -> phys 911000, readback OK
+[pit]  timer at 100 Hz
+[kbd]  ps/2 keyboard driver ready
+[sched] 4 threads: idle, shell, workerA, workerB
+
+[boot] enabling interrupts, starting preemptive scheduler
+[boot] workerA/workerB log from separate stacks to prove round-robin
+
+
+frostbyte shell ready. type 'help'.
+frostbyte> [workerA] tick=50 iterations=7143449
+[workerB] tick=91 iterations=15207163
+[workerA] tick=150 iterations=26517115
+[workerB] tick=191 iterations=35124050
+hel[workerA] tick=250 iterations=45724522
+p
+available commands:
+  help    show this list
+  echo    print the given text
+  mem     physical memory and heap usage
+  uptime  ticks and seconds since boot
+  ps      list kernel threads
+  clear   clear the screen
+frostbyte> [workerB] tick=291 iterations=53487849
+me[workerA] tick=350 iterations=64811083
+m
+physical memory: 130559 KiB total
+  frames: 32736 total, 2324 used, 30412 free
+  used 9296 KiB, free 121648 KiB
+heap: 48 KiB used of 8192 KiB
+frostbyte> [workerB] tick=391 iterations=73119504
+ps[workerA] tick=450 iterations=84004093
+
+id  state     name
+ 0   runnable  idle
+*1   running  shell
+ 2   runnable  workerA
+ 3   runnable  workerB
+frostbyte> [workerB] tick=491 iterations=92765972
+upt[workerA] tick=550 iterations=104540981
+ime[workerB] tick=591 iterations=112423170
+
+uptime: 601 ticks, 6 seconds (at 100 Hz)
+frostbyte> [workerA] tick=650 iterations=123652364
+ech[workerB] tick=691 iterations=132218480
+o fr[workerA] tick=750 iterations=140041519
+os[workerB] tick=791 iterations=147051909
+tbyt[workerA] tick=850 iterations=154503810
+e l[workerB] tick=891 iterations=160779592
+ives[workerA] tick=950 iterations=165666327
+
+frostbyte lives
+frostbyte> [workerB] tick=991 iterations=173432697
+[workerA] tick=1050 iterations=181711346
+[workerB] tick=1091 iterations=192171261
+[workerA] tick=1150 iterations=201288528