Fixup for (EXPERIMENTAL) FoundationDB support

Fixup for (EXPERIMENTAL) FoundationDB support

Better-defined generator semantics; many internal improvements,
a couple found bug fixes, minor feature enhancements.

Considerable simplification that streamlines and improves the
baseline performance of library internals. Updates tests and
example documentation.

  - Add inclusive()/exclusive() selector endpoints.
  - default is still: select { a, b } == [a, b).
  - endpoint semantics work for reads, pagination,
    range-erase, split planning.
  - tests: selector endpoint combinations; endpoint handling; multiple
    value types from generators..
  - runtime byte-pointer helpers now actually just runtime

Assisted-by: Codex:GPT-5
Signed-off-by: Jesse Williamson <jfw@ibm.com>