fix: make SLH scattering field concretely typed

[oameye]

SMatrix carries a hidden length parameter L (the total element count N*N), so declaring the field as SMatrix{N,N,ST} left L free and rendered the scattering field abstract. This was caught by the CheckConcreteStructs (Concretely typed) test set.

Adds L to the struct and an inner constructor that infers it from the matrix, preserving the existing SLH{N,ST,LT,HT}(...) calling convention so no call sites needed changes. Existing dispatch sites (SLH{N}, SLH{N,ST,LT,HT}) are partial type applications and still match.

[github-actions]

Benchmark Results

Details

Benchmark suite	Current: 7bdde5f	Previous: 6a468fd	Ratio
Correlations/two-time/single photon cavity	183182452 ns	183117800 ns	1.00
Interaction Picture/coefficient matrix M/analytical (2 equal modes)	13355 ns	13104 ns	1.02
Interaction Picture/coefficient matrix M/numerical (ODE)	73768 ns	73197 ns	1.01
Interaction Picture/coupling matrix evaluation/2 modes	95.8377239199157 ns	96.30126849894292 ns	1.00
Interaction Picture/coupling matrix evaluation/4 modes	202.41472868217053 ns	204.60943396226415 ns	0.99
Interaction Picture/operator substitution/TLS cascade	17703 ns	17773 ns	1.00
Pulse Couplings/multi-pulse/input 2 modes	415798 ns	414315 ns	1.00
Pulse Couplings/multi-pulse/output 2 modes	463096 ns	464319 ns	1.00
Pulse Couplings/single-pulse/input	368309 ns	369711 ns	1.00
Pulse Couplings/single-pulse/input Gaussian	89.03866248693835 ns	89.57306889352819 ns	0.99
Pulse Couplings/single-pulse/output	401572 ns	403004 ns	1.00
Pulse Couplings/single-pulse/output Gaussian	92.51048218029351 ns	90.81799163179916 ns	1.02
SLH Algebra/closure evaluation/2-QD waveguide H(t)	35567 ns	34375 ns	1.03
SLH Algebra/closure evaluation/2-QD waveguide L(t)	3363.875 ns	3233.5 ns	1.04
SLH Algebra/numeric/2-QD waveguide composition	30627 ns	21570 ns	1.42
SLH Algebra/symbolic/3-cavity cascade	6858.8 ns	7576.75 ns	0.91
SLH Algebra/symbolic/concatenate + cascade	2166.222222222222 ns	2702.777777777778 ns	0.80
SLH Algebra/symbolic/feedback OPO loop	5867 ns	6590.2 ns	0.89
Translation/closure evaluation/3-cavity H(t)	8673 ns	8616 ns	1.01
Translation/closure evaluation/3-cavity L(t)	8656 ns	9077 ns	0.95
Translation/static/atom-cavity	40406 ns	39484 ns	1.02
Translation/time-dependent/3-cavity H+L	616654 ns	596616 ns	1.03
Translation/time-dependent/atom-cavity	134461 ns	122599 ns	1.10

This comment was automatically generated by workflow using github-action-benchmark.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Benchmark Results'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.30.

Benchmark suite	Current: 7bdde5f	Previous: 6a468fd	Ratio
SLH Algebra/numeric/2-QD waveguide composition	30627 ns	21570 ns	1.42

This comment was automatically generated by workflow using github-action-benchmark.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.77%. Comparing base (8e7b370) to head (7bdde5f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
- Coverage   96.78%   96.77%   -0.01%     
==========================================
  Files           4        4              
  Lines         404      403       -1     
==========================================
- Hits          391      390       -1     
  Misses         13       13              

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