Unify the bond-dimension cap vocabulary on max_bond_dim: Option<usize> (today: max_bond_dim/max_rank/maxbonddim)

[shinaoka]

Problem

The bond-dimension cap has three spellings in the workspace:

• max_bond_dim: usize with a usize::MAX sentinel: tensor4all-simplett CompressionOptions
• max_rank: Option<usize>: tensor4all-treetn TruncationOptions (with_max_rank)
• maxbonddim: Option<usize>: tensor4all-quanticstci options, which translates its own field into its dependency's spelling (options.rs:287)

treetn and simplett each contain two of the three. Every user crossing crates relearns the name, and a user or agent arriving from ITensors.jl has no stable target for maxdim. This was identified as a vocabulary decision in the 2026-08-02 audit umbrella (#566, Phase 2); this issue splits it out because it does not depend on tensor4all-agent-rules#6 and now has an additional motivation (ITensors.jl persona mapping, see #573).

Decision

Adopt max_bond_dim: Option<usize> workspace-wide (None = unlimited):

• max_bond_dim over max_rank: rank is overloaded in TCI context; bond dimension is the unambiguous tensor-network term and the closest cousin of ITensors.jl maxdim.
• Option<usize> over a usize::MAX sentinel: idiomatic, and the unlimited case is explicit at call sites.

Proposed direction

1. simplett CompressionOptions: max_bond_dim: usize becomes Option<usize>; remove the sentinel.
2. treetn TruncationOptions: rename max_rank/with_max_rank to max_bond_dim/with_max_bond_dim.
3. quanticstci: rename maxbonddim to max_bond_dim; drop the translation shim once spellings match.
4. Early development policy applies (no backward compatibility, remove old names immediately); include a CHANGELOG mapping table old name to new name, plus the ITensors.jl row (maxdim maps to max_bond_dim).
5. Coordinate with the tolerance-vocabulary decision in Umbrella: 2026-08-02 origin/main audit remediation and shared-rules adoption #566 Phase 2 (SvdTruncationPolicy as the shared tolerance type); this issue deliberately covers only the bond cap and does not preempt that work.

Acceptance criteria

• grep -rn "max_rank\|maxbonddim" crates/ returns nothing outside CHANGELOG.
• One spelling and one type for the cap across simplett, treetn, quanticstci, and any options struct that forwards it.
• conventions.md (or the ITensors mapping documentation) states maxdim maps to max_bond_dim and cutoff maps to rtol*rtol.

[shinaoka]

Resolved by PR #599 (merged 7a6db77 base; issue #566 remediation follow-up).

• Workspace-wide max_bond_dim: Option<usize> (None = unlimited): max_rank -> max_bond_dim in core SvdOptions/TruncateOptions and treetn TruncationOptions (incl. builders/getters); maxbonddim was already renamed in PR3: API vocabulary, stack seam, and architecture (issue #566) #595.
• usize::MAX sentinel removed from public options: simplett CompressionOptions/ContractionOptions/FactorizeOptions, tensorci TCI2Options/conversion, treetci TreeTciOptions, AciOptions, partitionedtt PatchingOptions, interpolativeqtt, quanticstransform FourierOptions. RrLUOptions (tcicore, internal) intentionally stays usize.
• grep -rn "max_rank\|maxbonddim" crates/ returns nothing outside CHANGELOG.
• CHANGELOG.md mapping table + conventions.md ITensors mapping (maxdim -> max_bond_dim, cutoff -> rtol = sqrt(cutoff)).

Acceptance criteria met; closing.