fix: make Tie::File test suite pass

[fglock]

Summary

./jcpan -t Tie::File goes from 336 failures across 16 test files to all 4725 tests passing, with no regressions in the existing unit tests.

Six related bugs in tied-array dispatch and $. handling, all fixed in the Java runtime:

1. $. numeric reads stale zero after local $. + reads.
   ScalarSpecialVariable.set() was caching this.type=INTEGER, this.value=0 on the proxy. Readline mutates currentLineNumber directly, so the cache goes stale. Numeric comparison fast paths (arg.type==INTEGER → (int)arg.value) then read 0. Removed the sync so $. always delegates through getValueAsScalar(). This is why Tie::File's _check_integrity reported "spurious caching of record N" everywhere.

2. $. not incremented for multi-char $/.
   readUntilString only bumped currentLineNumber on \n in the line. With a custom separator containing no newline, $. stayed 0. Now increments once per record read, matching Perl.

3. Tied push / unshift returned the tie method's raw return value.
   Perl's av.c ignores the PUSH/UNSHIFT return and reports FETCHSIZE. Tie::File's PUSH returns nothing on purpose. tiedPush / tiedUnshift now call the handler and then return FETCHSIZE.

4. Tied SPLICE always called in scalar context.
   tieCall hardcoded SCALAR, so @r = splice(@tied, ...) got back the scalar count. Context now flows through Operator.splice and TieArray.tiedSplice to the user's SPLICE; the bytecode handler unwraps a tied scalar return directly.

5. Negative subscripts on tied arrays passed raw to FETCH / STORE.
   Perl normalizes negative indices (FETCHSIZE + idx) before dispatch. If the result is still negative (e.g. $tied[-100] on a size-3 array), Perl does NOT call FETCH / STORE: reads yield undef, writes throw Modification of non-creatable array value attempted, subscript -N. Implemented via an outOfRangeOriginalIndex flag on RuntimeTiedArrayProxyEntry, with matching normalization in exists / delete.

6. @tied = (...) didn't call EXTEND.
   Perl calls EXTEND on tied arrays before the STORE loop. Tie::File relies on this to extend the backing file in autodefer mode.

Test plan

• ./jcpan -t Tie::File — 4725/4725 pass (was 4389/4725)
• make — full unit test suite passes (no regressions)

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