feat: implement v0.1.0-beta.1 blockers

[CharlesHoskinson]

Summary

• Close 6 of 8 beta blockers (2 were already resolved on master), moving EARS score from 26/50 to 31/50
• REQ-LC-005: Remove dual runtime path — delete dosbox_step() and MachineContext::step(), update 9 test files
• REQ-TH-004: Add std::mutex to MixerState for thread-safe compound access from audio callback
• REQ-EX-004: Expand reentrancy guard (in_step check) to 11 additional API functions
• REQ-IN-004: Fix text_input atomicity — pre-check slot capacity before enqueueing any events
• REQ-SR-002: Serialize CPU GPR (EAX-EDI, EIP, EFLAGS, 6 segment registers) in save state, with backward-compat for V4 files
• REQ-SEC-038: Enforce 64 KB max shader file size in loadCustomShader()
• REQ-SEC-039: Enforce 256 MB soundfont and 1 MB ROM file size limits

Test plan

• Verify build passes on all 3 platforms (Windows, Linux, macOS)
• Run full test suite — all previously-passing tests should still pass
• Run TSan build to verify mixer mutex resolves data races
• Verify save/load round-trip preserves CPU register state
• Verify oversized shader/soundfont files are rejected