Phase 2b: convert Z80 micro-op methods to a class

Replace the dynamic 'z80.method = (value) => ...' pattern with a proper
class. Methods use 'this.*' for registers; flag tables and constants stay
in module scope (closure access, no 'this' indirection needed).

  class Z80 { constructor() {...}; inc(v){...}; and(v){...}; ... }
  export const z80 = new Z80();

Methods now live on Z80.prototype instead of as own properties, giving
the z80 object a cleaner hidden-class shape (just the register fields).
All external exports and lifecycle functions are unchanged.

All 1329 FUSE CPU tests pass.

🤖 Generated by LLM (Claude, via OpenClaw)

Author: mattgodbolt-molty