fix: render index slot suffixes as a comma subscript

[oameye]

Problem

An index accumulates numeric per-slot suffixes as the pipeline transforms it, via (i::Index)(k) (which does Symbol(name, "_", k)). For example an index reaches i_2_1 when completion mints a second distinct atom i(2) (i_2, slot 2) and scale later collapses it to its position-1 representative (i_2)(1) (i_2_1, slot 1).

_latex_index_suffix emitted the name verbatim as _{i_2_1}. The unbraced _2_1 is a double subscript, which MathJax rejects with "Double subscripts: use braces to clarify", so the entire equation was left unrendered (raw LaTeX source) in Documenter pages and notebooks.

Surfaced by the QuantumCumulants superradiant_laser_indexed example (corr_sc.eqs), while investigating QuantumCumulants PR #317.

Fix

The suffixes are slot numbers, so join them into a single comma subscript: i_2_1 renders as i_{2,1}. Bare index names (:i, :j) have no suffix and are unchanged.

The accumulated _2 part is load-bearing (it distinguishes the second-atom representative from the first, so two-atom moments do not fold into same-atom ones), so the name itself is preserved; only its rendering changes.

Tests

Added a regression testset in test/printing/printing_test.jl pinning i_{2,1} and confirming bare names are untouched. All 133 rendering tests pass.

Verified the transformed output renders in both MathJax v2 and v3.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.52%. Comparing base (e7e7648) to head (4077d66).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #213   +/-   ##
=======================================
  Coverage   94.51%   94.52%           
=======================================
  Files          27       27           
  Lines        2937     2939    +2     
=======================================
+ Hits         2776     2778    +2     
  Misses        161      161           

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[github-actions]

Benchmark Results

Details

Benchmark suite	Current: 4077d66	Previous: e7e7648	Ratio
Accumulation/Many-mode H/foldl M=16	11041 ns	10980 ns	1.01
Accumulation/Many-mode H/foldl M=24	24586 ns	24876 ns	0.99
Accumulation/Many-mode H/foldl M=8	2765.1111111111113 ns	2800.777777777778 ns	0.99
Accumulation/Many-mode H/sum M=16	1718.2 ns	1697.1 ns	1.01
Accumulation/Many-mode H/sum M=24	2294.222222222222 ns	2305.4444444444443 ns	1.00
Accumulation/Many-mode H/sum M=8	695.4850746268656 ns	729.7297297297297 ns	0.95
Accumulation/Same-site/foldl	6205.6 ns	6275.6 ns	0.99
Accumulation/Same-site/sum	2346.5555555555557 ns	2295.3333333333335 ns	1.02
Commutator/Nested JC/depth=1	1671.1 ns	1705.1 ns	0.98
Commutator/Nested JC/depth=2	18285 ns	17583 ns	1.04
Commutator/Nested JC/depth=3	66674 ns	66503 ns	1.00
Commutator/Nested JC/depth=4	170669 ns	168372 ns	1.01
Commutator/Nested JC/depth=5	378527 ns	372361 ns	1.02
Commutator/Schrieffer-Wolff/[S, V]	196577 ns	198388 ns	0.99
Commutator/Schrieffer-Wolff/[S, [S, H0]]	359101 ns	367321 ns	0.98
Indexing/Diagonal collapse/[H_Dicke, S_j]	43261 ns	44253 ns	0.98
Indexing/Diagonal collapse/[H_JC, σ_j]	62326 ns	63227 ns	0.99
Indexing/Simplify/double-sum spin-spin	16641 ns	16551 ns	1.01
Indexing/Simplify/indexed JC H	1150.2 ns	1169.1 ns	0.98
Indexing/Sum construction/double Σ_ij(J_ij*S_i*S_j)	8512.333333333334 ns	8766 ns	0.97
Indexing/Sum construction/single Σ_i(σ_i*σ_j)	11552 ns	10669 ns	1.08
Normal Order/Fock (c*c')^n/n=2	621.9545454545455 ns	611.7403314917127 ns	1.02
Normal Order/Fock (c*c')^n/n=3	894.2033898305085 ns	892.1754385964912 ns	1.00
Normal Order/Fock (c*c')^n/n=4	1198.2 ns	1183.2 ns	1.01
Normal Order/Fock (c*c')^n/n=5	1597.9 ns	1565.9 ns	1.02
Normal Order/Ground state/3-level rewrite	840.8 ns	809.5487804878048 ns	1.04
Normal Order/Multi-mode/2-mode 6-op chain	978.8 ns	964.421052631579 ns	1.01
Simplify/Jaynes-Cummings/H	1078 ns	1050 ns	1.03
Simplify/Jaynes-Cummings/H²	2431.222222222222 ns	2412.3333333333335 ns	1.01
Simplify/Two cavities/H	1113.1 ns	1103 ns	1.01
Simplify/Two cavities/H²	4497 ns	4495.428571428572 ns	1.00
Simplify/Λ-system/H	1343.5 ns	1352.5 ns	0.99
Simplify/Λ-system/H²	2284.222222222222 ns	2276.4444444444443 ns	1.00

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