refactor: rename two-stem complement output to backing across CLI, WASM, and web app

[Copilot]

Two-stem mode currently emits the complementary track as no_<source>, which is awkward in user-facing workflows and downstream handoff. This change standardizes the complement stem/output filename to backing while preserving the existing add/minus construction behavior.

• Output contract

  • Native CLI two-stem output now writes:
    • <source>.wav
    • backing.wav
  • WASM/Node CLI and browser separation path now use internal stem name backing instead of no_<source>.

• Web app UX alignment

  • Two-stem summary text now reflects backing.wav.
  • Result card label keeps context without changing the stem key:
    • Backing track (without <source>)
  • Downloaded file naming remains stem-key based (backing.wav).

• Docs and CI expectation updates

  • README two-stem example updated from no_bass.wav to backing.wav.
  • CI check for native two-stem artifact now expects backing.wav.
  • Related test expectation updated to match the new output naming.

// two-stem naming (WASM/Node and app paths)
const names = args.twoStems
  ? [args.twoStems, "backing"]
  : SOURCES;

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