feat(doubles): mock sequences and an assert_have_never_been_called

[Chemaclass]

Problem

bashunit::mock (src/doubles/mock.sh:45) replaces a command with a fixed body or a fixed exit code, and that is all it can do. Two everyday testing needs have no expression:

1. Different results per call. Retry logic, polling loops and backoff are exactly the code that most needs testing, and they all require "fail the first time, succeed the second":

# there is no way to write this today
bashunit::mock curl --returns-in-sequence 1 1 0

The workaround is a hand-rolled counter file in each test, which every user reinvents.

2. Asserting a command was never called. The spy side can assert "called N times", but expressing "this destructive command must never run" means writing assert_have_been_called_times 0 rm, which reads backwards and — before #895 — silently passed for a command that was never spied at all.

Proposal

bashunit::mock_sequence <cmd> <body-or-code> [<body-or-code>…]

• Each call consumes the next entry; the last entry repeats once exhausted (document this — the alternative, failing after exhaustion, is also defensible; pick one and test it).
• Entries follow the existing convention that an all-digits value is an exit code and anything else is a body (bashunit::doubles::is_exit_code, src/doubles/mock.sh:37), so the two doubles keep reading the same way.
• Registered in the same registry the runner unwinds after each test, so no leakage between tests.

assert_have_never_been_called <cmd>

• Fails when the command was called at least once, showing the recorded call log (bashunit::spy::call_log_to_slot, src/doubles/spy.sh:92).
• Fails when <cmd> was never spied, matching the fix(doubles): call assertions silently pass when the command was never spied #895 rule that an unregistered double is a broken test, not a vacuous pass.

Acceptance criteria

• bashunit::mock_sequence cmd 1 0 returns 1 on the first call and 0 on the second
• Behaviour after the sequence is exhausted is documented and tested
• Bodies and exit codes can be mixed in one sequence
• The sequence state resets between tests (no leakage), including under --parallel
• bashunit::unmock clears a sequenced mock completely
• A spy over a sequenced mock still records every call
• assert_have_never_been_called cmd passes when the command was spied and never called
• It fails, printing the call log, when the command was called
• It fails when cmd was never spied (fix(doubles): call assertions silently pass when the command was never spied #895 rule)
• The generated mock body stays free of injectable shell syntax (the reason is_exit_code exists — see the comment at src/doubles/mock.sh:33)
• Bash 3.0 compatible; no per-assertion fork regression
• Documented in docs/test-doubles.md; tests in tests/unit/doubles/ and tests/functional/doubles_test.sh

Repo checklist (agent)

• TDD: RED → GREEN → REFACTOR. Write the failing test first.
• Bash 3.0+ only: no printf -v, no += append, no declare -A, no [[ ]], no ${var,,}, no &>>, no ${arr[-1]}. Expanding a possibly-empty array under set -u needs ${arr[@]+"${arr[@]}"}.
• A new CLI flag must be wired in all of these or a parity test fails:
  • parse in src/main/test.sh (report-style flags need export -n, see src/main/test.sh:188-196 for why)
  • validate the value in bashunit::main::validate_config_or_exit (src/main/validate.sh:60) — unvalidated input used to run the wrong thing and exit 0 (Unknown options are silently ignored: a typo'd flag runs a different suite and exits 0 #871, --jobs with a non-integer value hangs on Bash 3.x and is silently ignored on Bash 4.3+ #873)
  • default + env var in src/config/env.sh and a documented line in .env.example
  • --help text in the same block it belongs to
  • both completions/bashunit.bash and completions/_bashunit (anti-drift test feat(cli): bash and zsh completion scripts with an anti-drift test #778 fails otherwise)
• Gates: make sa, make lint, ./bashunit tests/, ./bashunit --parallel tests/. Never run shfmt -w.
• Docs: update docs/command-line.md. Editing docs/assertions.md invalidates the bashunit doc acceptance snapshot — regenerate it.
• CHANGELOG.md: add one line under ## Unreleased.
• Fixtures under tests/acceptance/fixtures/ must not end in *test.sh.
• One issue = one PR.

[Chemaclass]

Done in #1070 (merged).