Add benchmark suite

[oameye]

Adds a benchmarks/ directory with BenchmarkTools.jl benchmarks covering the main features of the package:

• SLH Algebra: symbolic cascade/concatenate/feedback, numeric SLHqo composition, and closure evaluation (the ODE hot loop)
• Translation: static and time-dependent translate_qo, plus closure evaluation
• Pulse Couplings: single-pulse u_to_gu/v_to_gv and multi-pulse u_eff/v_eff
• Interaction Picture: coupling matrix A(t) evaluation, coefficient matrix M(t) solve (numerical + analytical), operator substitution
• Correlations: two-time correlation matrix

Benchmarks use realistic system sizes from the examples. Closure evaluation benchmarks specifically target the per-timestep allocation

5-element BenchmarkTools.BenchmarkGroup:
  tags: []
  "SLH Algebra" => 3-element BenchmarkTools.BenchmarkGroup:
          tags: []
          "symbolic" => 3-element BenchmarkTools.BenchmarkGroup:
                  tags: []
                  "feedback OPO loop" => TrialEstimate(1.664 ms)
                  "concatenate + cascade" => TrialEstimate(9.722 ms)
                  "3-cavity cascade" => TrialEstimate(49.348 ms)
          "numeric (SLHqo)" => 1-element BenchmarkTools.BenchmarkGroup:
                  tags: []
                  "2-QD waveguide composition" => TrialEstimate(70.020 μs)
          "closure evaluation" => 2-element BenchmarkTools.BenchmarkGroup:
                  tags: []
                  "2-QD waveguide L(t)" => TrialEstimate(45.750 μs)
                  "2-QD waveguide H(t)" => TrialEstimate(64.030 μs)
  "Interaction Picture" => 3-element BenchmarkTools.BenchmarkGroup:
          tags: []
          "operator substitution" => 1-element BenchmarkTools.BenchmarkGroup:
                  tags: []
                  "TLS cascade" => TrialEstimate(557.508 ms)
          "coefficient matrix M" => 2-element BenchmarkTools.BenchmarkGroup:
                  tags: []
                  "numerical (ODE)" => TrialEstimate(233.490 μs)
                  "analytical (2 equal modes)" => TrialEstimate(18.300 μs)
          "coupling matrix evaluation" => 2-element BenchmarkTools.BenchmarkGroup:
                  tags: []
                  "2 modes" => TrialEstimate(449.259 ns)
                  "4 modes" => TrialEstimate(3.720 μs)
  "Pulse Couplings" => 2-element BenchmarkTools.BenchmarkGroup:
          tags: []
          "single-pulse" => 2-element BenchmarkTools.BenchmarkGroup:
                  tags: []
                  "output" => TrialEstimate(1.523 ms)
                  "input" => TrialEstimate(1.329 ms)
          "multi-pulse" => 2-element BenchmarkTools.BenchmarkGroup:
                  tags: []
                  "output 2 modes" => TrialEstimate(2.709 ms)
                  "input 2 modes" => TrialEstimate(4.688 ms)
  "Correlations" => 1-element BenchmarkTools.BenchmarkGroup:
          tags: []
          "two-time" => 1-element BenchmarkTools.BenchmarkGroup:
                  tags: []
                  "single photon cavity" => TrialEstimate(405.307 ms)
  "Translation" => 3-element BenchmarkTools.BenchmarkGroup:
          tags: []
          "static" => 1-element BenchmarkTools.BenchmarkGroup:
                  tags: []
                  "atom-cavity" => TrialEstimate(458.006 μs)
          "closure evaluation" => 2-element BenchmarkTools.BenchmarkGroup:
                  tags: []
                  "3-cavity L(t)" => TrialEstimate(14.970 μs)
                  "3-cavity H(t)" => TrialEstimate(20.010 μs)
          "time-dependent" => 2-element BenchmarkTools.BenchmarkGroup:
                  tags: []
                  "3-cavity H+L" => TrialEstimate(3.239 ms)
                  "atom-cavity" => TrialEstimate(810.541 μs)
```julia