windows-msys: bare boot pins driver + getpath warnings, not exit status (round-9)

Run 34760958198's windows leg failed the bare scenario on its POSIX
assertions: the child printed 'unreached' and exited 0, with only the
driver warning and getpath's "Could not find platform independent
libraries <prefix>" on stderr.

Mechanism, established against the 3.14.7 sources: the encodings import
at startup is unconditional (pylifecycle.c _PyUnicode_InitEncodings ->
unicodeobject.c _PyCodec_InitRegistry, fatal on failure), so a child
that ran user code necessarily FOUND a stdlib. The channel is upstream:
Modules/getpath.py's winreg section (os_name == 'nt' and
use_environment and winreg) appends the HKCU/HKLM
SOFTWARE\Python\PythonCore\<ver>\PythonPath registry values to sys.path
— and the GHA runner has a registered host python. Bare boot on windows
is therefore environment-dependent by upstream design: exit status and
the encodings fatal are not assertable there.

The msys bare scenario now pins the deterministic surface instead: the
driver's bare-boot warning (ours, every platform) plus getpath's
platstdlib warning (the prefix fell back to the baked-in mount root,
which never exists on the runner). POSIX keeps the strict trio
(non-zero exit + driver warning + encodings fatal) — no registry
fallback exists there.

Author: tebako-ci

tools/boot_smoke

@@ -35,7 +35,11 @@
 #                        stdlib is image-resident (the two-artifact
 #                        model), so the driver warns about the bare
 #                        boot and the interpreter fails on its own
-#                        terms (non-zero exit; every platform)
+#                        terms (non-zero exit + the encodings fatal on
+#                        POSIX; on windows upstream's winreg fallback
+#                        can hand the child a host-registered stdlib,
+#                        so only the driver's warning and getpath's
+#                        platstdlib warning are pinned there)
 #   boot                 TEBAKO_RUNTIME_IMAGE handoff: stdlib imports
 #                        (json/zlib/ssl) off the mounted image, sys.prefix
 #                        == the mount root                       [POSIX]
@@ -341,18 +345,35 @@ def run_scenarios(executable, platform, selected, tfs:, line:) # rubocop:disable
       case scenario
       when "bare"
         # The two-artifact model: the exe alone is NOT a dev-mode
-        # interpreter — the stdlib lives in the env image, so a bare
-        # boot can only fail (getpath's compiled-in prefix resolves
-        # empty and the encodings import dies). What this scenario pins
-        # is the DRIVER's half of the contract: it names the bare boot
-        # on stderr, then the interpreter fails on its own terms.
+        # interpreter — the stdlib lives in the env image. What this
+        # scenario pins on every platform is the DRIVER's half of the
+        # contract: it names the bare boot on stderr.
         out, err, st = boot(executable, ["-c", "print('unreached')"], {})
-        check("bare: non-zero exit (the stdlib is image-resident)",
-              !st.nil? && !st.exitstatus&.zero?, "stdout: #{out}\nstderr: #{err}")
         check("bare: the driver names the bare boot",
               err.include?("TEBAKO_RUNTIME_IMAGE is not set"), "stderr: #{err}")
-        check("bare: the interpreter ran and died on the missing stdlib",
-              err.include?("encodings"), "stderr: #{err}")
+        if platform.msys?
+          # Windows startup is environment-dependent by upstream design:
+          # Modules/getpath.py ("os_name == 'nt' and use_environment and
+          # winreg") appends the HKCU/HKLM SOFTWARE\Python\PythonCore\<ver>\
+          # PythonPath registry values to sys.path, so on a host with a
+          # REGISTERED python (the GHA runner ships one) the child finds a
+          # stdlib, imports encodings, and RUNS the -c program — observed on
+          # the 0dc372c windows leg (run 34760958198): exit 0, 'unreached'
+          # printed. On a registry-clean host it dies with the encodings
+          # fatal like POSIX. Exit status and the encodings message are
+          # therefore not assertable here; the deterministic upstream
+          # surface is getpath's platstdlib warning (the prefix fell back
+          # to the baked-in mount root, which never exists on the runner).
+          check("bare: getpath names the missing platstdlib",
+                err.include?("Could not find platform independent libraries"), "stderr: #{err}")
+        else
+          # POSIX: no registry fallback — getpath's compiled-in prefix
+          # resolves empty and the encodings import dies at Py_Initialize.
+          check("bare: non-zero exit (the stdlib is image-resident)",
+                !st.nil? && !st.exitstatus&.zero?, "stdout: #{out}\nstderr: #{err}")
+          check("bare: the interpreter ran and died on the missing stdlib",
+                err.include?("encodings"), "stderr: #{err}")
+        end
       when "boot"
         unless File.file?(image)
           check("boot", false, "no sibling env image #{image}")