fix(expand_archive): normalize backslashes on Windows in markerInsideDestination (#15)

The marker-inside-destination prefix check was skipping the
backslash -> forward-slash canonicalisation when running on
Windows (it was guarded by `when not defined(windows)`). The
check then appended "/" to the destination before the prefix
test:

  m.startsWith(d & "/")

so a marker like `C:\actions-runner\config.cmd` was compared
against `C:\actions-runner/` and rejected with

  expandArchive.build: marker 'C:\actions-runner\config.cmd'
  must live inside destination 'C:\actions-runner'

The off-platform tests passed because their canonicalisation
made everything forward-slashes; the on-Windows path silently
broke every recipe that used backslash separators (which is
basically every Windows recipe).

Apply the same canonicalisation on every host. The existing
test "marker inside destination on Windows-style paths" now
actually exercises the Windows path (it always passed on
Linux but would have failed on Windows pre-fix).

Surfaced during L3 production-profile validation: with PRs #11
+ #12 + #13 + #14 the profile-compile chain reached the run
stage of system_windows_runner.nim, which calls expandArchive.build
with `destination = C:\actions-runner` and
`marker = C:\actions-runner\config.cmd`. That call now succeeds.

--no-verify: pre-existing prek migration-mode failure
(no .pre-commit-config.yaml in repo).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: zah

libs/repro_dsl_stdlib/src/repro_dsl_stdlib/packages/expand_archive.nim

@@ -252,15 +252,15 @@ proc markerInsideDestination*(marker, destination: string): bool =
   if marker.len == 0 or destination.len == 0:
     return false
   proc canon(p: string): string =
-    var lowered = p
-    when not defined(windows):
-      # On non-Windows hosts the input may still be a Windows-style
-      # path (when the test fixture mirrors a Windows recipe). We
-      # canonicalise backslash -> forward slash so the prefix check
-      # is direction-agnostic; the path itself stays unevaluated
-      # because we are not touching the filesystem here.
-      lowered = lowered.replace("\\", "/")
-    if lowered.len > 1 and lowered[^1] in {'/', '\\'}:
+    # Canonicalise backslash -> forward slash on EVERY host. On non-Windows
+    # the input may still be a Windows-style path (test fixture mirrors a
+    # Windows recipe); on Windows the input may be either separator — and
+    # the prefix check below appends "/", so leaving backslashes in place
+    # made an in-destination marker like ``C:\foo\bar`` look outside
+    # ``C:\foo``. The path itself stays unevaluated because we are not
+    # touching the filesystem here.
+    var lowered = p.replace("\\", "/")
+    if lowered.len > 1 and lowered[^1] == '/':
       lowered.setLen(lowered.len - 1)
     lowered
   let m = canon(marker)