Generate derived pure-Perl modules in XS fallback builds

[fglock]

Summary

Kossy 0.63 fails under PerlOnJava because the pure-Perl fallback build of Text::Xslate 3.5.8 omits the generated Text::Xslate::PP::Const module.

Evidence

• CPAN run: 20260907-214651-8410
• Distribution: Kossy 0.63
• PerlOnJava result: 4 subtests failed; the affected tests fail during module loading with Can't locate Text/Xslate/PP/Const.pm.
• System Perl result: all 21 Kossy test files pass, 92 tests total.
• Text::Xslate 3.5.8 upstream build derives lib/Text/Xslate/PP/Const.pm from src/xslate_opcode.inc in builder/MyBuilder.pm.

Expected behavior

When an XS distribution declares a pure-Perl fallback, the fallback build should run its required generated-file steps. Text::Xslate's pure-Perl implementation should then load successfully and Kossy's test suite should pass.

Failure details

PerlOnJava's fallback build reports Text::Xslate 3.5.8 built with 25 files, but Text/Xslate/PP/Const.pm is absent. Kossy's failures cascade from that missing generated module; this is not a Kossy assertion failure.

Suggested regression coverage

Add a focused build/test regression that runs Text::Xslate 3.5.8 through the PerlOnJava pure-Perl XS fallback and verifies that Text::Xslate::PP::Const can be loaded, followed by Kossy 0.63 compatibility coverage.