fix(toolchain): narrow the sweep, and stop spawning cl.exe to ask a yes/no

[Sunrisepeak]

Both found by reviewing the v2026.8.16.3 diff after tagging it.

1. The sweep could delete another version mid-install 🔴

sweep_parked_payloads walked the whole family directory and removed any version directory containing no regular files.

But an install populates a version directory over time — which is exactly why package_fetcher tracks completeness with a marker file rather than by existence. So a different version being extracted right now is briefly indistinguishable from a leftover skeleton:

xim-x-msvc/
  14.44.35207/        ← this command's skeleton, safe to sweep
  14.52.36629/        ← another process, 200ms into extraction, no files yet

Two mcpp processes against one MCPP_HOME is ordinary on a shared or self-hosted runner — and this whole park/sweep feature exists for long-lived installs on precisely those machines.

Fix: .trash-* is still swept family-wide (that name is only ever written by this code, so deleting one is safe whoever else is running). The file-less-skeleton rule now applies to the single version the command names.

2. A yes/no question was spawning a compiler

msvc_available_here() wants to know whether a usable toolset exists. It called the full installation_at(), which runs cl.exe and parses its banner to identify the version — information it then discards.

That question is asked on every build at the MSVC-ABI gate, so a machine with several installed toolsets paid several subprocess spawns per build. Those are the same machines the predicate was added for (managed toolsets, no Visual Studio).

Fix: installation_at(..., identifyVersion = false) skips the banner. The layout still has exactly one implementation — this is a parameter, not a second copy of the path arithmetic (which is the sin the review's §1 is about).

Testing

TheSweepDoesNotTouchAnotherVersionThatHasNoFilesYet was run against the family-wide sweep and fails there. Full suite 84/84.

Shipped in v2026.8.16.3; these go into the next release.