fix(e2e): 256 looked for env=, and the emitted spelling is env = "msvc"

The tar fix landed — both legs packed and the archive was written, which is what
that test exists to show: `mathkit.lib` beside `libmathkit.a` in one Windows
package. What failed after it was the predicate assertion, against a package that
was perfectly correct: the generated manifest writes

    [target.'cfg(all(arch = "x86_64", os = "windows", env = "msvc"))'.build]

and the grep looked for `env=`, which appears nowhere in the file. Matched with
optional whitespace now.

A grep that can only fail is worth more attention than one that passes, because
it reads as a product bug on the one platform where nothing else could confirm
it. Audited the other emitted-manifest assertions in the suite for the same
shape; the rest either allow spacing (`role *= *`) or match structural text.

Author: speak-agent

tests/e2e/256_pack_library_fat_windows.sh

@@ -90,9 +90,12 @@ grep -qE "^\[target\.'$ARCH-" "$pkg/mcpp.toml" && {
     cat "$pkg/mcpp.toml"
     echo "FAIL: a leg is selected by a BARE TRIPLE, which is inert on a native build"
     exit 1; }
-# env= must appear: `os=windows` alone cannot separate these two legs, and a
-# predicate that cannot separate them would hand MSVC consumers the MinGW archive.
-grep -q "env=" "$pkg/mcpp.toml" || {
+# An env axis must appear: `os = "windows"` alone cannot separate these two legs,
+# and a predicate that cannot separate them would hand MSVC consumers the MinGW
+# archive. Matched with optional spaces — the emitted spelling is `env = "msvc"`,
+# and the first version of this grep looked for `env=`, which is nowhere in the
+# file and failed against a package that was perfectly correct.
+grep -qE 'env[[:space:]]*=' "$pkg/mcpp.toml" || {
     cat "$pkg/mcpp.toml"
     echo "FAIL: the predicates carry no env axis, so both legs match both targets"
     exit 1; }