Lower the FFT op family in the native Expr compiler

[ausimian]

Nx.fft/ifft (1-D, trailing axis) and the fft2/ifft2/rfft/irfft
blocks now lower under compiler: Emily.Compiler, native: true instead of
forcing a graceful fallback to the evaluator. Same shape as the
take_along_axis addition: the MLX ops and eager NIFs already existed
(Native.{fftn,ifftn,rfftn,irfftn} → mlx::core::fft::*); only the
compiler path was missing.

What surfaced this

Running a Whisper speech_to_text serving with native: true logged:

Emily Expr compiler does not yet lower op :fft (no fallback). It will
be added in a later milestone. The defn ran op-by-op via the evaluator.

That's the log-mel featurizer's STFT. With this change that defn compiles
fully native. (The whisper_full conformance native lane never caught it
— it tests the encoder/decoder forward on pre-computed features, not the
featurizer.)

Changes

• c_src/emily/opcodes.hpp — add Fftn/Ifftn/Rfftn/Irfftn
  (88–91), bump kOpcodeCount to 92, dispatch each to the matching
  mx::fft::* with FFTNorm::Backward (unnormalized), mirroring the eager
  fft.cpp entry points. operands [input]; iattrs [[sizes...],[axes...]].
• lib/emily/ir.ex — add the opcodes; lower :fft/:ifft (lower_op)
  on the trailing axis (mirroring Emily.Backend.{fft,ifft}/3, which use
  the last axis and ignore opts[:axis], so native stays bit-identical to
  the evaluator); lower Nx.Block.{FFT2,IFFT2,RFFT,IRFFT} (lower_block)
  with the block's sizes/axes. Output coerced to out.type (complex64 for
  the forward transforms, real for irfft) like the backend wrap.
• compiler_equivalence_test.exs — native-vs-evaluator bit-identical
  cases for fft/ifft (+ explicit length, batched), fft2/ifft2, rfft/irfft.

Verification

• New FFT equivalence cases pass under native: true (test config defaults
  native_fallback: :raise, so a fallback would raise — passing proves
  full native lowering). Opcode-parity test confirms the IR↔C++ enum
  lockstep (88–91 / kOpcodeCount 92).
• mix precommit green: compile (warnings-as-errors), format, credo
  --strict, docs, 686 tests, 0 failures.

Resolves the :fft gap noted while trying the native compiler on the
Whisper livebook.