Fix reduction sink root-relative paths

[imlvts]

This should address #142

Changes:

• Added sink_template_prefix to derive write roots from the sink template rather than the entire sink expression.

• Fixed both fixed and ignored-variable paths in:

  • CountSink
  • HashSink
  • AndSink
  • SumSink
  • FloatReductionSink

  Each now strips wz.root_prefix_path() before calling move_to_path.

• Reused the template-prefix logic for PureSink.

• The optional WASMSink had the same unstripped output-path issue, plus stale method signatures that prevented the feature from compiling. Its resource handling and root-relative output path are now corrected.

[imlvts]

Relevant speed regression tests

Benchmark	Before	After	Difference
odd_even_sort mean	11.403 s	11.459 s	+0.056 s (+0.5%)
odd_even_sort median	11.415 s	11.450 s	+0.035 s (+0.3%)
taxi_lts mean	102.7 ms	104.7 ms	+2.0 ms (+1.9%)