test(fused): restore fused.rs coverage above threshold

[Ryo-wtnb11]

Summary

strided-kernel/src/fused.rs is below the 80% per-file coverage threshold on
main (currently ~77% in CI), so the coverage job fails on every PR. This
adds the missing tests to bring it back over the line.

Cause

The complex FusedScalar implementation (impl_complex_fused_scalar!) had no
test coverage at all — every existing fused test uses f64. Real
negate / conj / abs were also the only real scalar ops not reached by the
existing op chains.

Change

Test-only. No production code changes.

• specializes_every_op_over_complex: single-instruction plans iterating every
  FusedOp over Complex64. Single-op plans always hit
  try_static_specialization, and both the static and interpreter paths
  dispatch through the scalar methods, so this exercises the whole complex impl.
  Inputs use positive real parts / nonzero operands so divide / log /
  sqrt / pow stay well defined, and static output is checked against the
  interpreter.
• specializes_real_negate_conj_abs: covers the remaining real scalar ops.
• validate_plan_rejects_out_of_range_output / validate_plan_rejects_zero_outputs
  / is_injective_layout_rejects_rank_and_broadcast_mismatch: cover the
  plan/layout validation error branches the positive tests skip.

Result

fused.rs line coverage 80% → 92% (locally). cargo test and
cargo fmt --check pass; scripts/check-coverage.py reports all files meet
their thresholds.

[Ryo-wtnb11]

Heads up: #140 currently cherry-picks this same commit (304147d) so its coverage check can go green ahead of this PR landing — main is below the per-file threshold on fused.rs until this merges. Once this merges, rebasing #140 onto main drops the now-duplicate commit automatically (git detects the identical patch). This PR remains the standalone, reviewable coverage fix.