feat: close phase 5 joypad and serial timing

[pakitovic]

Summary

• close the remaining Phase 2 bridge work needed before input/peripheral closure
• complete Phase 5 joypad and serial behavior on the shared T-cycle scheduler
• retain phase-specific timing artifacts and update the handbook/roadmap docs

What changed

• verify and harden the EI ; HALT pending-IRQ edge and add the missing MMIO-oriented CPU load/store forms
• finish joypad JOYP visibility, visible-edge IRQ generation, and STOP wake ordering through the shared interrupt/scheduler path
• finish serial SB/SC ownership, bit-level transfer timing, peer boundary, completion IRQ timing, and scheduler-visible trace coverage
• add retained Phase 5 trace fixtures for joypad+STOP and externally clocked serial progress
• refresh affected synthetic ROM fixtures and golden traces

Validation

• make check