docs: bash-on-Windows is proven broken (WSL as local system), with the log

The 58f4832 Windows run completed and failed, but not on the bootstrap: its
Windows steps still used `shell: bash`, and on winvm-builder bash resolves to
WSL, which refuses to run under the runner's service account:

  Running WSL as local system is not supported.
  Error code: Bash/WSL_E_LOCAL_SYSTEM_NOT_SUPPORTED
  ##[error]Process completed with exit code 1.

The step died on the shell without ever invoking cmake. So the PowerShell
rewrite in 524e78c -- which was written pre-emptively from reading the
workflow back, on the reasoning that bash "cannot be assumed present" -- is
now confirmed necessary by a real log rather than by argument. It is still
unproven in the other direction: no completed run has yet included it.

Equally important for reading the record correctly: this failure says nothing
about the OBS::w32-pthreads blocker. That result stands on edb0c02 alone, and
the README now says so explicitly so a later reader does not count 58f4832 as
corroboration.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RL8abRmgFXkVASHkkqiJbE

README.md

@@ -310,10 +310,13 @@ pushes leaves a queue that takes an hour to drain.
 failed.** The first 7 failures were all at commits predating the `-A x64` fix
 below. The 8th, at `edb0c02` — the first commit that actually carries that fix
 — has since completed, and it is the informative one: **the `-A x64` fix
-worked, and Windows failed further along, on something else.** The PowerShell
-rewrite is still queued behind it and remains unproven. Do not read either fix
-below as "confirmed"; check current CI status rather than trusting this
-paragraph's age.
+worked, and Windows failed further along, on something else.** A later run
+(`58f4832`) then failed for an unrelated reason — its Windows steps still used
+`shell: bash`, which on this runner is WSL and cannot run as local system —
+so it never reached cmake at all and tells us nothing about the bootstrap.
+The PowerShell rewrite that fixes that is still queued and remains unproven.
+Do not read any fix below as "confirmed" beyond what is stated; check current
+CI status rather than trusting this paragraph's age.
 
 What `edb0c02` showed: obs-deps and Qt6 downloaded, CEF skipped, the OBS
 sub-configure ran for 53s and correctly reported "Plugin Support" and "User
@@ -415,10 +418,22 @@ getting past it, the second is still unproven:
 
    Plain `-A x64` now. **Confirmed fixed:** the `edb0c02` run got past this
    and downloaded `windows-deps-2023-11-03-x64.zip` correctly.
-2. The Windows CI steps were originally written in bash (via
-   `shell: bash`), which is a poor fit for a `windows-latest` runner's
-   default toolchain expectations; they were rewritten in PowerShell. **Still
-   unproven** — no completed run has included it.
+2. The Windows CI steps were originally written in bash (via `shell: bash`)
+   and were rewritten in PowerShell. **The bash version is now proven broken**
+   by the `58f4832` run: on `winvm-builder`, `bash` resolves to WSL, and WSL
+   refuses to run under the service account the runner uses —
+
+   ```
+   Running WSL as local system is not supported.
+   Error code: Bash/WSL_E_LOCAL_SYSTEM_NOT_SUPPORTED
+   ##[error]Process completed with exit code 1.
+   ```
+
+   so the step died on the shell without ever invoking cmake. The PowerShell
+   rewrite is therefore necessary, but **still unproven in the other
+   direction** — no completed run has yet included it. Note this also means
+   `58f4832`'s failure says nothing about the `w32-pthreads` blocker; that
+   result comes from `edb0c02` alone.
 
 Until a Windows run completes green, Windows should be treated as unverified
 beyond "the core library and the WinHTTP backend compile and their tests