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feat(lns): benchmark suite + design doc for add/sub algorithms (Phase D of #777) (#787)

* feat(lns): benchmark suite + design doc for add/sub algorithms (Phase D of #777)

Close out the configurable lns add/sub roadmap (Epic #777) with the two
deliverables from issue #782:

1. benchmark/performance/arithmetic/lns/log_add_algorithms.cpp
   Per-algorithm throughput + accuracy benchmark across representative
   (nbits, rbits) configurations: (8,4), (16,8), (24,16), (32,16). Times
   each policy via direct Alg::add_assign / sub_assign calls (bypassing
   the traits dispatch so all 5 algorithms can be measured in the same
   translation unit). Accuracy is sampled over a geometric sweep of
   operand magnitudes spanning the lns dynamic range with both same-sign
   and mixed-sign pairs, against DirectEvaluation as the oracle. Output
   is a Markdown table per config, suitable for direct paste into
   release notes.

   Empirical observations (illustrative, host-dependent):
   - Throughput differential between algorithms is small (~1 M ops/sec
     on a desktop x86_64 build) because the lns class encode/decode
     dominates the per-call cost. The differential at the sb_add(d)
     level on raw double operands -- the regime hardware co-processors
     actually pipeline -- is much larger but not exposed by the lns
     class API benchmark.
   - Accuracy matches the analytical bounds: Lookup ~9.5e-5 rel error
     at default IndexBits on rbits=16, Polynomial ~1.1e-5 rel,
     ArnoldBailey caps at ~5e-2 rel.

2. docs/design/lns-add-sub.md
   Design document explaining the framework. Covers:
   - Why log-domain addition is the hard case (Gauss log-add formulation)
   - The lns_addsub_traits<Lns> customization point + rationale for the
     traits-class pattern over a fifth template parameter
   - The shared detail::gauss_log_add<Policy> dispatcher and per-algorithm
     contract (sb_add, sb_sub)
   - Per-algorithm description, error model, and use case for each ... (continued)

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