Translate cpu detection from assembly to C++

[tinix0]

Summary

Translates detproc.asm, the last hand-written x86 assembly CPU-detection
routine, to C++, folding it directly into getcpu.cpp alongside the rest of
Get_CPU_Type. While doing so, this simplifies MMX and CMOV detection to
assume both are always present rather than probing CPUID feature bits, since
the project's supported minimum hardware (SSE2, so a Pentium 4 or Athlon 64
onward) always carries them. Detect_CMOV_Availability and Processor are
removed outright, as neither had a remaining caller once that assumption held.
Get_CPU_Clock (previously #if 0'd out in mpu.cpp) is enabled using the
__rdtsc() intrinsic in place of the inline _emit opcode bytes it used
before.

A new tests/cpudetect CTest target builds the ported detection standalone
and checks its reported family, vendor, and feature flags against CPUID read
directly. No gameplay-visible change on any machine
meeting the documented minimum hardware.

Behavior and compatibility

• Preserved: the CPUID-derived processor family and vendor string
  (Get_CPU_Type's cpu_type/vendor output, the CPUType/VendorID
  globals) — the family is still read from CPUID leaf 1 exactly as the
  assembly did.
• Intentional behavior change (breaking): UseMMX, UseCMOV, and
  HasCMOV are now fixed-true constants instead of being written by CPUID
  feature-bit checks, and Detect_CMOV_Availability/Processor are removed.
  These globals still gate the MMX/CMOV instruction paths in winasm.asm's
  palette-fade routines, so a machine below the documented minimum hardware
  (SSE2, so a Pentium 4 or Athlon 64 onward) would now execute an instruction
  it lacks instead of falling back to the plain path — such hardware was
  never in the supported build matrix, so this formalizes an existing
  requirement rather than narrowing one.
• Not a compatibility boundary: no changes to save/replay/network
  formats, INI/data formats, or COM/ABI surfaces — this is process-internal
  engine plumbing.
• Version and migration: targets release 0.2.0 (current development
  release). Migration steps are recorded in
  manual/changes/cpu-feature-detection.md: drop any code calling
  Detect_CMOV_Availability or Processor, and treat UseMMX/UseCMOV/
  HasCMOV as fixed-true constants rather than runtime-detected flags.

Validation

• Manual build smoke tested on GDI 1
• CTest ensures all the fields are set when calling the ported methods

Documentation

• Added manual/changes/cpu-feature-detection.md (breaking: true, release
  0.2.0) documenting the MMX/CMOV assumption, the removed functions, and the
  migration steps.
• Updated inline comments in getcpu.h, getcpu.cpp, mpu.h, mpu.cpp,
  init.cpp, and misc.h to state the modern-hardware assumption and remove
  stale references to the now-removed Detect_CMOV_Availability, Processor,
  and the deleted DETPROC.ASM.

Checklist

• The change is focused; unrelated mechanical cleanup is separate
• Compatibility effects and any migration are explicit
• A player- or modder-visible engine change carries its change record
• Validation distinguishes what passed, failed, and was not run
• No prohibited assets, binaries, SDKs, credentials, or generated output are included

[github-actions]

Development builds of 96ae888:

• opents-Release-bc74046
• opents-Debug-bc74046

The links work without a GitHub account. Artifacts expire after 90 days, and this comment follows the latest successful build.