51. First-Character Dispatch in the VMs (Instruction Set 3)

• Changes 49 and 50 in the VMs: a new instruction, GUARD c, d, l, is emitted before the CHOICE of an alternative that must begin with a given terminal (the analysis, firstTerminal, is shared by every backend). The instruction-set version of compiled files is now 3; files of versions 1 and 2 still load (they simply have no guards).
• Effect (min of 10 interleaved runs, Apple M3 Max): JSON 18.5 → 16.1 ms (recursive VM, −13%) and 22.4 → 19.0 ms (iterative VM, −15%); Minilang −8% on both; Arith_OperatorsRule −4% and −7%; XML −1% and −7%.