tests: decode console utf-16le stderr in capture_stderr helpers

dynamic_bad_tag_warns_once failed (count 0) ONLY in console-attached
singleproc runs (user report; reproduced deterministically with stderr
on a real console and stdout redirected): when stderr is backed by a
windows console channel, a pushed chan transform receives the console
driver's utf-16le wide-char byte stream - every ascii char arrives
NUL-interleaved, so content regexps never match. The punk::args warning
itself emits correctly, exactly once, inside the capture window (proven
with marker writes: markers and warning all captured, all wide).
Piped/file stderr and -jobs child processes deliver plain bytes, which
is why every piped run passed.

Fix in all three capture_stderr helpers (punk/args dynamic.test,
punk/ansi grepstr.test, punk/ns corp.test - identical latent defect):
if the captured data is NUL-interleaved, decode it as utf-16le
(fallback 'unicode' for tcl 8.6) before returning. Verified: the
console-attached args subtree run is green (246/243/3/0); piped runs
unchanged.

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

src/tests/modules/punk/ansi/testsuites/ansi/grepstr.test

@@ -70,6 +70,15 @@ namespace eval ::testspace {
         if {$code} {
             return -options $ropts $r
         }
+        if {[string first \x00 $caught] >= 0} {
+            #stderr backed by a windows console: the pushed transform receives the console
+            #driver's utf-16le wide-char byte stream rather than encoded bytes (each ascii
+            #char arrives NUL-interleaved), so content assertions never match - decode.
+            #(found 2026-07-19 via the sibling helper in punk/args dynamic.test failing in
+            #console-attached singleproc runs; piped stderr and -jobs children are unaffected)
+            set enc [expr {"utf-16le" in [encoding names] ? "utf-16le" : "unicode"}]
+            set caught [encoding convertfrom $enc $caught]
+        }
         return $caught
     }
 

src/tests/modules/punk/args/testsuites/args/dynamic.test

@@ -34,6 +34,15 @@ namespace eval ::testspace {
         if {$code} {
             return -options $ropts $r
         }
+        if {[string first \x00 $caught] >= 0} {
+            #stderr backed by a windows console: the pushed transform receives the console
+            #driver's utf-16le wide-char byte stream rather than encoded bytes (each ascii
+            #char arrives NUL-interleaved), so content assertions never match - decode.
+            #(found 2026-07-19: dynamic_bad_tag_warns_once failed ONLY in console-attached
+            #singleproc runs; piped/file stderr and -jobs child processes deliver plain bytes)
+            set enc [expr {"utf-16le" in [encoding names] ? "utf-16le" : "unicode"}]
+            set caught [encoding convertfrom $enc $caught]
+        }
         return $caught
     }
 

src/tests/modules/punk/ns/testsuites/ns/corp.test

@@ -198,6 +198,15 @@ namespace eval ::testspace {
         if {$code} {
             return -options $ropts $r
         }
+        if {[string first \x00 $caught] >= 0} {
+            #stderr backed by a windows console: the pushed transform receives the console
+            #driver's utf-16le wide-char byte stream rather than encoded bytes (each ascii
+            #char arrives NUL-interleaved), so content assertions never match - decode.
+            #(found 2026-07-19 via the sibling helper in punk/args dynamic.test failing in
+            #console-attached singleproc runs; piped stderr and -jobs children are unaffected)
+            set enc [expr {"utf-16le" in [encoding names] ? "utf-16le" : "unicode"}]
+            set caught [encoding convertfrom $enc $caught]
+        }
         return $caught
     }
     #sp n - n plain spaces (keeps expected values reviewable)