fix(filters): scope source filters per compilation unit

[fglock]

Summary

Source filters installed via Filter::Util::Call::filter_add lived on a thread-local global stack and the wasFilterInstalled flag was never reset between files. As a result, a filter installed while compiling file A leaked into any file A require'd.

This broke jcpan -t URI::git:

• Test/Base.pm does use Spiffy -Base;, which calls Spiffy::spiffy_filter to install a regex filter that prepends my $self = shift; to every sub.
• Spiffy's import then calls Exporter::export(...), which lazily requires Exporter::Heavy.
• While Heavy.pm was being parsed, the Spiffy filter was still active and got applied to it, mangling heavy_export and producing the spurious "my" variable $self masks earlier declaration warning at Exporter/Heavy.pm line 237.
• With heavy_export broken, field was never installed in Test::Base, and the next line field _filters => [qw(norm trim)]; parsed as two consecutive barewords and died with syntax error near "=> [qw".

Fix

Treat the source-filter stack and the "installed during use" flag as part of the per-file compilation context:

• FilterUtilCall.java: add FilterStateSnapshot, saveAndResetFilterState(), restoreFilterState().
• ModuleOperators.java: snapshot and clear the filter state before compiling a require/do'd file and restore it in finally, so filters installed by the caller cannot leak into the required file (and vice versa).

Test plan

• jcpan -t URI::git passes all 3 subtests (was: syntax error in Test::Base)
• make (full unit-test suite) green

Generated with Devin

[fglock]

Subsumed by #597 — unified into a single PR.

#597 adopts this PR's cleaner design (wire-up at ModuleOperators.do_file rather than PerlLanguageProvider.executePerlCode, and the FilterStateSnapshot / saveAndResetFilterState / restoreFilterState naming). Earlier I had wired into executePerlCode and needed a skipSaveRestore carve-out for preprocessWithBeginFilters to avoid regressing perl5_t/t/op/incfilter.t from 143/153 to 14/153. This PR's placement at do_file sits outside executePerlCode and so is naturally untouched by the recursive executePerlCode call inside preprocessWithBeginFilters — no carve-out needed. Cleaner.

Verified after refactor:

• ./jperl src/test/resources/unit/source_filter_scope.t 4/4 (with fix; 1/4 with fix neutralised — bug correctly caught)
• perl5_t/t/op/incfilter.t 148/153 (no regression from master baseline)
• make and ./gradlew test (CI's task) green
• make test-bundled-modules: all 34 YAML tests pass

#597 also includes:

• regression test src/test/resources/unit/source_filter_scope.t
• bundling of real YAML.pm 1.31 into src/main/perl/lib/ (so the YAML test suite runs against real YAML.pm semantics rather than YAML::PP)
• 34 upstream YAML-1.31 tests added under src/test/resources/module/YAML/t/
• two related fixes in do CODEREF / do FILEHANDLE: STORE no longer called on user's tied $_, and __FILE__ now resolves to the stringified GLOB/CODE
• design doc updates in dev/design/source_filters.md