compilers: add MSVC

[maci0]

Adds compilers/MSVC.md documenting the preserved Microsoft Visual C++ line (VC 1.0–11.0) and links it from the compilers index.

The page covers, in the style of the existing compiler pages:

• Versions — the 16-bit Windows 3.x line (VC 1.0/1.5/1.52, Phar Lap DOS-extender, DOSBox) vs the 32-bit line (VC 2.0–11.0), with the full CL.EXE version matrix and the VC 6.0 service-pack story (12.00.8168 RTM→SP3, 12.00.8804 SP4+, SP1 payload not preserved).
• Running MSVC — no native Linux build; wine/wibo for 32-bit, DOSBox for 16-bit, packaged as self-contained docker images (rebrew/msvc:<version>-<arch>) with a REBREW_RUNNER runtime switch; decomp.me hosts a subset under wibo.
• Identifying an MSVC compiled binary — the version-detection heuristics: Rich header + linker version (pins exact SP), pre-6.0 linker-version mapping, msvcpX.dll imports, PDB S_COMPILE3, codegen fingerprints (/O1 vs /O2 wrappers, pre-6.0 constant-caching), diec.
• Known quirks — the C1.DLL/C2.DLL WINEPATH trap, NE header offset vs Borland, TimeDateStamp determinism.
• Where to get it — archaic-msvc / archaic-toolchains preservation orgs, decomp.me msvcwin9x releases, archive.org / WinWorld media.
• Projects — decomp.me, rebrew's documented targets.

Facts are sourced from the rebrew/rebrew-toolchains docs (pinned sha256 manifests, provenance tables) and this session's verification.