fix: preserve plain-string semantics for %vd formatting

[fglock]

Summary

%vd formats an ordinary dotted numeric string as a version vector instead of
as the string's character code points.

Reproducer

printf "%vd\n", "1.22.333";

Expected behavior

Perl 5.42 prints:

49.46.50.50.46.51.51.51

Each character in the plain string is formatted as a decimal code point.

Actual behavior

Both the JVM and interpreter backends print:

1.22.333

Scope confirmed during investigation

This is specifically a plain string that happens to match a dotted-numeric
shape. A true v-string is correctly rendered as 1.22.333 by both Perl and
PerlOnJava, while adding a nonnumeric suffix makes PerlOnJava use the normal
character-vector path:

printf "%vd\n", v1.22.333;    # expected and actual: 1.22.333
printf "%vd\n", "1.22.333x"; # expected and actual: 49.46.50.50.46.51.51.51.120

Likely cause

SprintfVectorFormatter currently treats every scalar whose string value
matches \\d+(\\.\\d+)* as a version vector. The formatter should use the
scalar's v-string/version representation, not infer version semantics from an
ordinary string's text.

Impact

Any code using %vd to serialize or inspect character data gets silently
incorrect output when the data is composed only of dot-separated digits. This
is particularly risky because the output is plausible-looking and no error is
raised.

Acceptance criteria

• An ordinary string "1.22.333" formats as character code points with %vd.
• A real v-string continues to format as 1.22.333.
• Verify both JVM and interpreter backends.