awk: fix GNU runtime parity in vendored goawk

[ewhauser]

Summary

• fix vendored goawk field splitting so each record keeps the split mode active when it was read, including restoring FS after FIELDWIDTHS or FPAT assignments
• default three-argument gensub calls to the live current record ($0) and honor the DST component in mktime datespecs
• add GNU awk conformance coverage and direct regression tests for the repaired behaviors

Root Cause

• lazy field parsing was consulting mutable FIELDWIDTHS and FPAT state instead of preserving the split mode that applied when the record was read
• the native gensub helper had no access to the interpreter's current record, so omitted targets silently operated on an empty string
• mktime parsed the DST datespec field but never applied it when resolving timestamps in local time zones

Validation

• make lint
• go test ./contrib/awk -count=1
• env GBASH_CONFORMANCE_AWK="$(./scripts/ensure-awk.sh)" GBASH_CONFORMANCE_BASH="$(./scripts/ensure-bash.sh)" GBASH_RUN_AWK_CONFORMANCE=1 go test ./contrib/awk -run TestAWKConformance -count=1

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