[pmeems]

Phase 0 of the CMake migration plan: audit, decisions and parity tooling. No CMake targets yet.

Motivation

CMake must reproduce what MSBuild really does, including implicit defaults, and "no lost flags / no new warnings" must be measured rather than assumed. This PR records the baseline that every later CMake phase is compared against.

Changes

• docs/CMAKE-MIGRATION.md: per-target sources, definitions, include directories, real link dependencies (hidden today by VcpkgAutoLink), generated MIDL files, custom build steps, runtime deployment, architecture-specific settings, hard-coded bin\<Config>\<Platform> paths, prerequisites, and decisions D1–D6.
• build/scripts/Compare-CompileCommands.ps1: reads compile and link lines from a detailed MSBuild log (builds the .slnx itself when no log is given) and, optionally, from a CMake compile_commands.json plus a verbose build log. It normalises paths and option order, diffs per translation unit and per binary, counts unique warnings and summarises dumpbin /dependents /exports /headers. MSBuild is found through -MSBuild, MWG_MSBUILD, PATH, then vswhere.
• .gitignore: out/, cmake-build-*/, CMakeUserPresets.json. The CMake vcpkg tree vcpkg_installed/cmake-<Platform> is already covered by vcpkg_installed/.
• docs/BUILDING.md: short pointer to the migration doc and the parity script.

Key decisions

• D1 CMake output goes to <binaryDir>/bin and <binaryDir>/lib, never to <repo>/bin. Preset dirs are short (out/b/x64d, ...). Scripts get an optional -BinDirectory later.
• D2 CMake uses its own vcpkg tree per platform (vcpkg_installed/cmake-<Platform>), with triplets pinned in the presets.
• D3 A single MapWinGIS.ActiveX.Interface target owns MIDL.
• D4 Option (a): the six shared ActiveX sources are compiled twice, as today.
• D5 The C# host gets a conditional ProjectReference (MwgCMakeBuild).
• D6 Parity is measured with the script above.

Audit findings that correct the plan: no project defines WIN32, _WINDOWS or _USRDLL, so the CMake defaults get dropped. MSBuild adds _WINDLL to DLLs and /JMC in Debug. Every binary embeds a manifest (/TLBID:1). MIDL also gets /W1 /char signed /target "NT60". The MSBuild tests depend on gmock.dll only because of auto-linking.

Baseline (MSBuild, VS 18.11, MSVC 14.51, SDK 10.0.28000.0)

	Debug x64	Release x64
TUs / links	42 / 4	42 / 4
Warnings	0	0
OCX exports	5 COM entry points incl. DllInstall	same
DLL characteristics	dynamic base, NX, high-entropy VA	+ CFG
Native tests	173 run, 170 passed, 3 skipped	—

The Win32 baseline is collected in the phase that first builds Win32 with CMake.

Dependencies

Based on development. First PR of the CMake stack.