docs: fix opcode table errors, kbd@ byte-store semantics, drop TODOs

docs: fix opcode table errors, kbd@ byte-store semantics, drop TODOs

- virtual machine.org: correct divide stack effect (n1 n2 -- n1/n2,
  was inverted as n2/n1), fix subtract wording (subtract top item from
  second), document comparison truth convention (-1 = true, 0 = false,
  signed) — clears all 4 dangling TODO markers from the opcode table.
- opcodes_00_09.org: document that kbd@ writes only the low byte of
  the pushed cell (upper bytes are stale stack memory), note the host
  DOS/BIOS keyboard-buffer drain side effect, clarify num advances IP
  5 bytes total, state that drop leaves the value in memory, and add
  the XMS unlock/free step to the halt cleanup list.