Enforce strict Einstein index rules in @tensor

[lkdvos]

Fixes #288.

@tensor silently accepted expressions where an index label appears more than twice, e.g.

@tensor T[-1 -2 -3; -4 -5] := A[-2 1] * B[1 -3; 1 -5] * C[-1 1; -4]

The only arity check lived in insertcontractiontrees! and counted getindices per factor.
Since getindices returns open indices, a label traced away inside one factor became
invisible and could be reused by the other factors. The check also never ran for products of
fewer than three factors, nor for a single tensor.

This adds verifyindices, a single pass over each statement that enforces the documented rule:
within one additive term every label appears once (open) or exactly twice (contracted), counting
all occurrences including traces. Parentheses group factors but do not open a new label scope;
different terms of a sum, different statements and explicit tensorscalar arguments do.
Violations throw an ArgumentError at macro expansion time. The pass runs as a preprocessor
right after normalizeindices (so primed labels compare equal) and replaces the partial check in
contractiontrees.jl; it also covers labels appearing more than twice in a single tensor and
terms of a sum with differing open indices, which previously surfaced as confusing runtime
IndexErrors.

Note this rejects code that used to run: three @test_throws IndexError cases in
test/tensor.jl / test/cutensor.jl now fail at expansion instead and were updated. Scanning
the @tensor calls in TensorKit, MPSKit, MPSKitModels, PEPSKit and TensorKitManifolds turned up
no violations on current main. It does flag PEPSKit v0.3.0's right_projector, where a trace
label was reused across factors -- a bug that had to be found and fixed by hand in
QuantumKitHub/PEPSKit.jl@9b43dad, and which this check would have caught at macro expansion.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 98.61111% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/indexnotation/verifiers.jl	98.61%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/indexnotation/contractiontrees.jl	100.00% <ø> (ø)
src/indexnotation/parser.jl	93.90% <ø> (+1.21%)	⬆️
src/indexnotation/verifiers.jl	95.23% <98.61%> (+6.69%)	⬆️

... and 4 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[VictorVanthilt]

I'm not familiar at all with programming macros and working with expressions so perhaps @Jutho best review this.

[Jutho]

Looks good to me. Was this logic completely Claude generated? Not bad.

[lkdvos]

Yeah, it mostly is, although I did steer it a little and had to clean up some things in the end.