feat: lower 19 more unary ops in the native Expr compiler

[ausimian]

Summary

Closes the largest cluster on the Expr op-coverage checklist (#188): 19 unary ops that the IR didn't lower yet but Emily.Backend already implements eagerly. Each was a silent-fallback cliff in user code that used trig / numeric-stability / bitwise / complex-component / NaN-detection ops.

• Direct-mapped (17 ops, new IR opcodes 94–110, bit-identical to the Evaluator) — expm1, tan, sinh, cosh, acos, asin, atan, acosh, asinh, atanh, round (decimals=0, matching Backend), bitwise_not, is_nan, is_infinity, conjugate, real, imag. Each routes to the same mx::* primitive as the eager unary NIF (c_src/ops/unary.cpp), so the native single-NIF path and the Evaluator land on identical bits.
• Composed (2 ops, no new opcode) — erfc = 1 - erf and cbrt = sign(x) * abs(x)^(1/3). MLX has no primitive for either; the eager Emily.Backend composes them from existing ops and the IR lowerer now does the same, so the bit pattern stays exact too.

After this PR the op-coverage probe (scripts/expr_op_coverage.exs) drops from 37 → 18 misses; the only remaining unary ops are count_leading_zeros / population_count, which are already documented as "no MLX primitive — unlowerable by design" in #188.

Test plan

• mix precommit clean (701 tests, 79 properties, 40 doctests, 0 failures).
• mix run scripts/expr_op_coverage.exs confirms all 19 ops now lower under native_fallback: :raise (down from 37 misses to 18, all 19 targeted ops moved from MISS → OK).
• New equivalence tests in test/emily/compiler_equivalence_test.exs cover:
  • direct-mapped float trig (expm1, tan, sinh, cosh, atan, asinh);
  • inverse-trig with domain-valid inputs (asin/acos/atanh over |x| < 1, acosh over x >= 1);
  • round over tie-break inputs (-1.5, -0.5, 0.5, 1.5, 2.3, -2.7);
  • composed erfc and cbrt over positive, negative, and zero inputs;
  • bitwise_not over s32/s64/u8/u32;
  • is_nan / is_infinity over a tensor mixing finite values, :infinity, :neg_infinity, and :nan (asserting the {:u, 8} predicate dtype after the trailing coerce);
  • conjugate/real/imag over a {:c, 64} input (asserting the {:f, 32} real-component out.type).
• Opcode parity test (test/emily/opcode_parity_test.exs) green — kOpcodeCount bumped 94 → 111 in lockstep with the Elixir @opcodes map and the C++ enum.
• Conformance lanes (:native, :fuse) — out of scope, no new lowerings expected to reach them.