feat: lower atan2, quotient and logical_xor in the native Expr compiler

[ausimian]

Summary

Closes the binary/compare cluster on the Expr op-coverage checklist (#188). Three ops, two new IR opcodes, one composite clause — all bit-identical to the Evaluator.

• atan2 — slots into @arith_binary as atan2: :arctan2. The shared handler casts both operands to out.type then emits the binary op (same as Emily.Backend's @renamed_arith_binary). New IR opcode arctan2 (111) dispatches to mx::arctan2.
• quotient — joins @arith_binary as quotient: :floor_divide. Emily.Backend.quotient/3 is exactly floor_divide(astype(a, out.type), astype(b, out.type)), which is what the shared handler already does with the new floor_divide opcode (112 → mx::floor_divide).
• logical_xor — no MLX primitive; the lowerer mirrors Emily.Backend.logical_xor/3 directly: (a != 0) != (b != 0), three not_equal emits over a pair of per-dtype scalar zeros, then coerce to {:u, 8}.

Probe (scripts/expr_op_coverage.exs) drops 18 → 15 misses; the binary/compare section of #188 is now empty.

Test plan

• mix precommit clean (704 tests, 79 properties, 40 doctests, 0 failures — three new equivalence tests).
• Op-coverage probe shows all three moved from MISS → OK.
• New equivalence tests in compiler_equivalence_test.exs:
  • atan2 over the four quadrants and the axis boundaries (±0, ±pi, ±pi/2) so the result lands on every special case the eager Backend hits.
  • quotient over s32/s64 (signed, positive and negative dividends) and u8/u32 (including the u8 max boundary). Both paths route through mx::floor_divide so they match the Evaluator bit-for-bit.
  • logical_xor over the four truth-table corners for both f32 and s32, asserting the {:u, 8} predicate dtype after coerce.
• Opcode parity test green — kOpcodeCount bumped 111 → 113 in lockstep with the Elixir @opcodes map and the C++ enum.

Follows the same pattern as #189.