docs: the MSVC ecosystem round, and the tool that made it decidable - #445

Two things worth keeping after the work is merged.

The report

Eleven defect layers, each reachable only once the one before it was fixed, every one of them green in CI at the time:

installed() only checked cl.exe  →  checked directories, not files
  → no kernel32.lib              →  no um/shared headers, no rc/mt
  → mcpp accepted a headers-only SDK
  → the build linked; remove failed on Windows
  → the error did not name the file
  → vctip.exe held the payload open
  → not installing it ≠ fixing machines that already had it
  → renaming the directory (wrong premise)
  → an empty skeleton would not delete either

One shape throughout: an acceptance criterion weaker than "usable". The endpoint is the one criterion that could not be met by adjusting a test — a real Windows runner completing the whole lifecycle, plus xrgui building 54/54 on the pinned Insider toolset with the vswhere workaround still deleted.

It also records what I got wrong, because those were the expensive parts:

• a premise about renaming directories on Windows that CI disproved
• a "which file is stuck" probe that found out by deleting
• an invented gcc@system proposal that ran against the design it claimed to improve (feat(toolchain): gcc@system is rejected, and says what to do instead #441, closed)
• three occasions where the honest conclusion was "this test cannot fail on the platform that would run it, so do not write it"

The tool

tools-msimap.py resolves an MSI's File/Component/Directory tables to real install paths, offline (7z for the OLE streams, !_Columns for the schema — the SDK's MSIs do not share column widths, and a guessed width silently misaligns every column after it).

It is here because it was the only thing that could answer "what is actually inside this payload". The package names, Microsoft's documentation, and my own comments in this repository were each wrong about the Windows SDK — Windows SDK Desktop Headers x64 ships four files, none of them a Win32 header — and only the tables were right.