Gate the native Whisper speech_to_text serving end-to-end - #186

A `cond`/`if` whose branches return a tuple crashed the native compiler
with a FunctionClauseError: the :cond lowering passed the tuple branch
straight to lower_node/2, which only matches a single tensor. Worse, a
FunctionClauseError escapes the compiler's graceful-fallback rescue (which
only traps ArgumentError), so it hard-faulted instead of degrading to the
evaluator. Surfaced by a Whisper encoder forward under native: true
(transpose(elem(cond -> {f32[1,1500,6,64] x N}, i))).

Lower a tuple cond to one where-chain per leaf position — identical
wholesale-select semantics to the single-output cond (the predicate is a
whole-tensor scalar bool; every branch is still computed) — and return a
{:multi_refs, [...]} handle that :elem projects, shared across sibling
:elems via lower_node's memo (like while). A nested / non-tensor container
raises a clean ArgumentError, so it falls back gracefully rather than
crashing.

- ir.ex: split the :cond clause (single-tensor head unchanged; new
  is_tuple head for the multi-output case); extend :elem to project
  {:multi_refs, refs}; add the tensors?/1 guard.
- compiler_control_flow_test.exs: tuple if + multi-clause tuple cond with
  mixed-shape (vector + scalar) leaves, native-vs-evaluator bit-identical.

Add a :whisper_full case that runs the real openai/whisper-tiny
speech_to_text serving on synthetic audio under native_fallback: :raise
— a no-fallback gate over the WHOLE graph: the mel featurizer's STFT
(fft), the encoder/decoder forward, and the autoregressive decode loop
(the multi-output cond, indexed_put cache writes, dynamic slices). An
un-lowerable op raises rather than silently degrading.

The existing whisper forward mode_test never reaches these — it feeds
pre-computed mel features through a single Axon.predict, exercising
neither the featurizer nor the generation loop. That blind spot is
exactly where this session's fft / indexed_put / tuple-cond gaps lived;
this locks the composed path so it can't silently regress. Reuses the
checkpoint whisper_full_test already downloads (no new fetch).

Also adds the RELEASE.md "Fixed" note for the tuple-cond crash and the
"Whisper now fully native end-to-end" milestone.