Add command and function introspection helpers to lib_std.sh

[codeforester]

Context

lib_std.sh already has fatal assertion helpers such as assert_command_exists, but callers sometimes need non-fatal command lookup or function availability checks for library/plugin-style preconditions.

Scope

Add Base-native command/function introspection helpers to lib_std.sh. Do not copy external implementations.

Acceptance Criteria

• Add a non-fatal command path helper using a caller-provided result variable.
• Add assert_function_exists for required Bash function checks.
• Function checks use Bash-native function introspection such as declare -F.
• Helpers preserve stdout unless a result output is intentionally requested through a variable.
• Invalid result variable names fail clearly.
• Public API and behavior are documented in lib/bash/std/README.md.
• BATS coverage verifies commands, builtins or PATH commands as appropriate, existing functions, missing functions, and invalid result names.

Validation

• Focused stdlib BATS coverage.
• ./tests/validate.sh
• `git diff --check