Provide a collision-resistant command runner API

[codeforester]

Context

The stdlib exports a public run helper. It is ergonomic, but run is a collision-prone name in Bash ecosystems, especially with BATS, shunit2, and caller-local wrappers. The test suite already preserves BATS' run as bats_run, which proves the conflict is real.

Scope

• Decide the canonical public name for the command runner, such as std_run or lib_run.
• Keep backward compatibility for existing callers if appropriate, possibly through an opt-in or compatibility alias.
• Update docs, examples, and tests to show the preferred API.
• Document the migration path for existing run users.

Acceptance Criteria

• The preferred command runner name is less likely to collide with external frameworks.
• Existing callers are either preserved or given a clear migration path.
• README.md, lib/bash/std/README.md, examples, and tests agree on the API.

Validation

• ./tests/validate.sh

Suggested Branch

enhancement/<issue>-20260618-command-runner-api