Guard CBE bond expansion against zero-content bonds

[lkdvos]

Problem

OptimalExpand and RandExpand (changebond!, and the infinite/multiline OptimalExpand loops) form the two-site update projected onto the state's orthogonal complement and then call svd_trunc!(normalize!(g2)). When a bond has no expansion content — a product-state region, or a symmetry sector the Hamiltonian does not couple into yet — g2 is exactly zero, so normalize! divides by a zero norm and manufactures NaNs. The subsequent SVD then throws:

ArgumentError: invalid argument #4 to LAPACK call

This is easy to hit with CBE on a low-entanglement or charged symmetric state. It surfaced in single-site CBE-TDVP (TDVP(; alg_expand = OptimalExpand(...))) on a charged fermionic (fℤ₂-graded) single-particle state: interior bonds there carry a single parity sector, so the projected update on them is structurally zero. (A merely small-but-nonzero g2, e.g. from a random dense state, does not trigger it — the norm must be exactly 0, which is why only structured/graded states hit it.)

Fix

Guard the zero-content case: when the projected update has (near-)zero norm there is nothing to expand at that bond, so skip it and leave the state untouched.

• Finite changebond! (OptimalExpand/RandExpand, both sweep directions): early-return when norm(g2) ≤ eps^(3/4).
• Infinite / multiline OptimalExpand loops: normalize only when the norm is above threshold.

SketchedExpand already handled this — it has an upfront empty-complement guard and orthogonalizes with QR/right_orth! rather than normalize!-then-SVD — so it is unchanged.

The guard only adds an early return on an exactly-zero-norm condition, so it cannot affect the normal (nonzero) expansion path.

Testing

• New regression test test/algorithms/changebonds.jl → "CBE zero-content bonds (graded)": a charged single-particle fℤ₂ state (interior bonds carry a single sector ⇒ structurally-zero expansion content) run through OptimalExpand and RandExpand; both must complete without crashing and preserve the state. This reproduces the crash on the unpatched code.
• Full test/algorithms/changebonds.jl suite is green (54/54).
• Verified end-to-end that OptimalExpand CBE-TDVP, which previously crashed on the charged fermionic state, now runs and matches TDVP2/RandExpand, while a full-rank sanity case confirms the guard does not suppress genuinely-needed expansion.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/algorithms/changebonds/randexpand.jl	50.00%	3 Missing ⚠️

Files with missing lines	Coverage Δ
src/algorithms/changebonds/optimalexpand.jl	98.85% <100.00%> (+0.11%)	⬆️
src/algorithms/changebonds/randexpand.jl	63.63% <50.00%> (-0.75%)	⬇️

... and 8 files with indirect coverage changes

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