Document and normalize the public Bash API

[codeforester]

Summary

• document the complete public API for the stdlib and tighten signatures and
  status contracts across every module README
• normalize private logging and Git implementation helpers to the
  double-underscore convention without changing public function names
• correct the list-library test path and add validation that every module README
  documents an existing BATS suite
• align top-level library summaries with the current canonical Git, GitHub, and
  repeatable argument APIs

Fixes #171

Validation

• ./tests/validate.sh (301 BATS tests)
• bats lib/bash/std/tests/lib_std.bats lib/bash/git/tests/lib_git.bats (187 tests, one environment skip)
• shellcheck --severity=error on production, validation, and launcher files
• public-function inventory checked against module READMEs
• git diff --check