test(dev): shorten Bash 3.2 feedback in sandboxed development #632

Goal

Catch GNU Bash 3.2.57 incompatibilities during local, sandboxed development
before a pull request reaches the GitHub Actions Bash 3.2 compatibility gate
or a developer encounters a failure on macOS.

This complements, rather than replaces, the existing CI gate. That gate builds
and authenticates Bash 3.2.57 from source and remains the authoritative
independent check until local/CI parity is established.

Context

The repository supports the macOS system Bash version, 3.2.57. Important
compatibility failures are often runtime-semantic rather than syntax errors;
for example, empty-array expansion with set -u behaves differently in Bash
3.2 than in newer Bash releases. ShellCheck and syntax parsing are valuable but
cannot prove this class of behavior.

The current GitHub Actions workflow runs the offline Bats suite under Bash
3.2.57, but its feedback arrives only after a push. Faster local feedback is
especially valuable when development happens inside a Linux sandbox whose
default Bash is newer.

Proposed Developer Interface

Provide one repository-owned command, for example:

scripts/test-acq-bash32 [test/bats/file.bats ...]

It must:

1. Resolve an explicit Bash 3.2.57 interpreter and fail closed if its version
   differs.
2. Prepend that interpreter's directory to PATH so Bats and nested
   #!/usr/bin/env bash scripts also use Bash 3.2.
3. Run the existing vendored Bats suite.
4. Accept focused Bats-file arguments for rapid iteration.
5. Offer a fast syntax-only mode, or a companion command, based on
   bash-3.2 -n over relevant host-side shell files.

Suggested usage:

devenv shell -- scripts/test-acq-bash32 test/bats/72-msb-agent-user.bats
devenv shell -- scripts/test-acq-bash32

Provisioning Options

Evaluate, rather than preselect:

• Project-pinned devenv / Nix provisioning of Bash 3.2.57.
• Mise as an optional launcher for the project-owned devenv command.
• An explicit ACQ_BASH32=/path/to/bash override for CI and pre-provisioned
  environments.
• Reuse of the existing source-build/signature-verification approach when Nix
  is unavailable.

The selected mechanism must pin its Nix input or equivalent package source and
verify the resolved interpreter version. It must not make ordinary acq run
depend on Nix, Mise, or Bash 3.2.

Validation Levels

The feature must document that it validates Bash interpreter compatibility, not
BSD/macOS utility portability such as differences in sed, find, stat,
xargs, or filesystem behavior.

Acceptance Criteria

• A documented sandbox-local command runs selected Bats files under verified
  Bash 3.2.57.
• The command can run the full offline suite under the same interpreter.
• Nested Bats and env bash invocations resolve to Bash 3.2.
• Non-3.2 interpreters fail with an actionable message.
• Interpreter provisioning is pinned and does not modify user-project runtime
  behavior.
• The existing GitHub Actions Bash 3.2 gate remains unchanged until parity is
  demonstrated.

Non-Goals

• Immediate replacement of the CI source-build/signature-verification flow.
• A claim that Linux Bash 3.2 reproduces BSD/macOS external-command behavior.
• Adding Bash 3.2, Nix, Mise, or devenv as a requirement for end-user
  workspaces.

AI-assisted research and drafting; human implementation and review required.