steve-downey / fingertree / 29168198708

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Import finger tree container and Paper C material

Lift the persistent measured sequence (finger_tree5) and its "Paper C"
capability set from steve-downey/trees into the beman split source layout,
renamed from smd:: to beman::fingertree.

Imported into include/beman/fingertree/:
- tree/: finger_tree5 container, iterator, PMR/AllocatorAware layer,
  foldable/traversable adapters, and the four capability wrappers
  (RandomAccess, Rope, PriorityQueue, IntervalIndex)
- typeclass/: the supporting typeclass chain (monoid, dual_monoid, functor,
  applicative, foldable, traversable, typeclass_base)
- fixpoint/: overloaded

Tests (Catch2, *.test.cpp) cover each component plus the self-enforcing
zero-global-allocation PMR proof and a compile-only regression probe. The
finger_tree2 cross-checks were dropped; finger_tree2 is not imported.

Build/deps:
- depend on beman.optional (lockfile.json, vcpkg.json, port, exported config)
- require C++26: finger_tree5 uses std::inplace_vector with no fallback

CI (ci_tests.yml) trimmed to C++26-capable toolchains: gcc >= 16 and clang
with libstdc++ (libc++ lacks <inplace_vector>). AppleClang, MSVC, lower
standards, and the C++ modules jobs are removed -- the beman.optional
dependency is not import-std clean, so the module cannot be built.

1009 of 1069 new or added lines in 13 files covered. (94.39%)

1012 of 1073 relevant lines covered (94.32%)

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