Isolate getopts state and preserve batch PATH order

[codeforester]

Problem

add_to_path and safe_mkdir reset and mutate the shell-global OPTIND instead of localizing parser state. A caller with OPTIND=9 observes OPTIND=2 after either helper parses one option, which can break subsequent caller-owned getopts processing.

add_to_path -p also reverses a batch: add_to_path -n -p /first /second produces /second:/first:<old-path> even though append mode preserves input order.

Scope

• Make helper option parsing use a local OPTIND and leave caller state unchanged.
• Preserve caller argument order for both append and prepend batches.
• Keep duplicate removal and strict/non-strict directory checks unchanged.

Acceptance Criteria

• OPTIND has the same value before and after add_to_path and safe_mkdir on success and failure.
• Prepending /first /second produces /first:/second:<old-path>.
• Duplicate and missing-directory behavior remains documented and tested.

Validation

• bats lib/bash/std/tests/lib_std.bats
• ./tests/validate.sh
• git diff --check

[codeforester]

Implemented by merged PR #172. add_to_path and safe_mkdir preserve caller OPTIND, and prepend batches retain input order. The regression is covered by add_to_path preserves caller OPTIND and batch prepend order and safe_mkdir preserves caller OPTIND.