Dynamic-strand bundle solver

DsBundle provides three persisted solver modes. Existing and missing scene data defaults to Legacy.

• Legacy preserves the original random bundle projection schedule.
• CoupledXpbd replaces cross-strand averaging with pairwise 6-DOF XPBD. Off-center midpoint anchors produce both translation and torque, while quaternion-log constraints correct relative bend and twist.
• Hybrid adds live cross-section rigid fitting and coarse longitudinal slice edges before the pairwise solve. This is the intended mode for solid, wood-like bundles and remains opt-in.

pair_iterations, coarse_iterations, compliance scales, and shape-matching strength are live controls. Slice spacing and minimum slice membership change buffer topology and require dynamic-strand reinitialization. Compliance scales are dimensionless multipliers over the existing material modulus graphs and segment geometry; they do not replace authored wood properties.

Hybrid slices use the static branch/node and root-distance bin together with the latest connectivity group_index. Cutting or structural failure therefore separates slice fits and coarse edges on the next Physics() call. Pair and coarse cohesion continuously weakens with connection integrity, carbon/lignin health, rot density, and moisture. A zero-integrity connection contributes no correction.

The board experiment is the controlled torque/rotation fixture. The large-log experiment covers scale, damage, and performance. Capture bundle diagnostics records rigid-fit rotation, center/far-side motion, constraint RMS, momentum residuals, dispatch count, owned strand/bundle memory, and the most recent 120 GPU profiler frames. Hybrid GPU time includes topology rebuild, slice fit/apply, coarse-edge solve, and pair solve/gather.

Current implementation limits:

• Hybrid topology uses GPU bitonic sorts followed by serial GPU range/edge reduction. This avoids CPU readback but is a correctness-first path that may be slower than Legacy for large logs.
• Components below minimum_slice_members or with ill-conditioned endpoint covariance fall back to pairwise XPBD.
• DsStiffRod continues to own per-strand longitudinal mechanics; coarse edges couple adjacent bundle slices.
• Legacy remains the default until board/log Vulkan-validation captures and timestep-variation measurements pass on a supported runtime configuration.