concurrency.test: retrying workspace cleanup for background-child log race

The two child-spawning tests (two-writers, lock-holder) proceed on the
children's done-marker files, but a just-finished child can still hold
its redirected stdout log open for a beat after writing the marker - on
Windows the -cleanup tree delete then hits 'permission denied'
(observed as a ~1-in-3 flake under the slower Tcl 8.6 interp, exposed
by the new native-8.6 runtests support; same race existed under Tcl 9).
cleanup_workspace retries the delete briefly (10s deadline) instead of
flaking. 8/8 consecutive clean runs under 8.6.17 post-fix; Tcl 9.0.3
punkcheck subtree 84/84.

Assisted-by: harness=claude; primary-model=claude-fable-5; api-location=anthropic.com

src/tests/modules/punkcheck/testsuites/punkcheck/concurrency.test

@@ -68,6 +68,18 @@ namespace eval ::testspace {
             after 25
         }
     }
+    #cleanup for workspaces where background children were spawned with redirected logs: the
+    #done-marker appears before the child fully exits, so on Windows its log file can still be
+    #open (undeletable) for a beat - retry the tree delete briefly instead of flaking
+    proc cleanup_workspace {root} {
+        set deadline [expr {[clock milliseconds] + 10000}]
+        while {[catch {file delete -force $root} errdel]} {
+            if {[clock milliseconds] >= $deadline} {
+                error "cleanup_workspace '$root': $errdel"
+            }
+            after 100
+        }
+    }
 
     #added 2026-07-21 (agent, G-095) - concurrency suite: atomic saves, advisory event lock, stale-break, non-interactive recovery
 
@@ -157,7 +169,7 @@ namespace eval ::testspace {
             list parse_errors [llength $parse_errors] reader_ran [expr {$observed_loads > 0}] installer_problems $dup_or_missing missing_targets $missing_targets leftovers $leftovers
         }\
         -cleanup {
-            file delete -force [dict get $ws root]
+            cleanup_workspace [dict get $ws root]
         }\
         -result {parse_errors 0 reader_ran 1 installer_problems {} missing_targets {} leftovers {}}
 
@@ -194,7 +206,7 @@ namespace eval ::testspace {
             list errcaught $errcaught code_ok $code_ok names_holder [string match "*held by:*HOLDER-CHILD*" $errmsg] lock_cleared [expr {![file exists $pf.lock]}]
         }\
         -cleanup {
-            file delete -force [dict get $ws root]
+            cleanup_workspace [dict get $ws root]
         }\
         -result {errcaught 1 code_ok 1 names_holder 1 lock_cleared 1}