Fix file section marker semantics

[codeforester]

Summary

• Count managed-section markers only when they match whole lines.
• Make update_file_section -r remove only the first matching section, matching update behavior.
• Document the exact-line and first-match behavior.

Validation

• shellcheck --severity=error lib/bash/file/lib_file.sh lib/bash/file/tests/lib_file.bats
• bats lib/bash/file/tests/lib_file.bats
• ./tests/validate.sh

Fixes #76
Fixes #78