[frankslin]

Address the code-review findings on the merged --ambiguities feature
(#1461) without changing the record schema or any output downstream
consumers already depend on.

Detailed Changes:

• Correctness:
  • Route the --ambiguities branch of ConvertFileStreams through the
    shared fclose epilogue instead of early-returning; the early return
    leaked both streams and broke --in-place, which replaces the output
    file after this function and requires it to be closed. Covered by a
    new in-place CLI test.
  • Apply PrefixMatch's group policy semantics in the walk via a
    recursive helper instead of calling the virtual Dict::MatchPrefix on
    groups: a DictGroup constructed directly with the Union policy
    reports Union (honored by Convert()'s PrefixMatch tables) while its
    inherited MatchPrefix still short-circuits, so the walk could emit
    different output than Convert() for directly constructed groups.
  • Mirror SingleStageConverter::Convert's no-segmentation branch by
    walking the whole text as one string_view, preserving embedded NUL
    bytes that the Segments/strlen round-trip silently truncated.
  • Propagate AnnotatedConversion::analyzed into AmbiguityStream::Chunk
    so stream consumers can distinguish "no ambiguities" from
    "converter not analyzable".
  • Detect mid-stream read errors (ferror) in ConvertAmbiguitiesStream
    and fail without emitting the end record, which is the stream's
    integrity signal and previously marked truncated streams complete.
• Streaming windowing:
  • Extract the flush-window computation shared verbatim by
    ConverterStream and AmbiguityStream into the private
    StreamWindow.hpp helper so a windowing fix reaches both wrappers;
    neither installed header changes.
• CLI measurement:
  • Count the JSONL bytes actually written (and the end record) in
    measurement.outputBytes, consistent with every other output mode,
    and time the end-record write; the end record's own output_bytes
    field keeps its converted-text semantics.
• npm CLI parity:
  • Reject --ambiguities with the same native-CLI guidance as
    --inspect/--segmentation, list it in the help text, and cover both
    in node/test.js, per the documented-and-tested rule for CLI
    behavior differences.
• Cleanup:
  • Replace Compose's unreachable trailing-run folding with asserts
    that both alignment sides exhaust together.
  • Add unit tests for the Union-policy group, NUL preservation, and
    the streamed analyzed flag.