fix(runner): a quoted shell-error phrase is no longer a runtime error

[Chemaclass]

🤔 Background

Related #992

detect_runtime_error decided whether a failing test also hit a shell error by scanning its captured output for phrases like command not found. That capture includes bashunit's own rendering of the failure, so any message quoting one of those strings was misread and the test reported twice — once as Failed, once as Error — for a single cause.

It's reachable without touching the framework. A test whose subject is error handling naturally has these strings in its output: asserting a script prints a useful message, capturing stderr from a failed command. Passing tests were never affected, which is why it stayed invisible until a test started failing — exactly when clear reporting matters.

💡 The discriminator

The prefix bash puts on its own diagnostics:

/tmp/rd2.sh: line 2: /etc/passwd: Permission denied

Checked across command-not-found, unbound variable, permission denied, no such file, syntax error, division by 0, readonly variable and integer-expression-expected, from inside a sourced test file. All carry : line N: . bashunit's own output never does.

Two groups, because they're prefixed for different reasons. Parser and expansion diagnostics require the prefix on the same line as the phrase. killed, segmentation fault and cannot allocate memory come from job control rather than the parser and can arrive without it, so they keep the whole-capture match — and none appears in bashunit's own output, so they carry no false-positive risk.

⚠️ The first attempt was wrong

Applying the prefix rule to everything broke two existing contracts — one pinning "process: killed", one pinning the extracted message's shape. Both were right to fail. The extraction still runs over the whole capture, not the matched line, so the message is byte-identical to before.

🔓 Lifts the #991 workaround

assert_true couldn't say "command not found" without triggering the duplicate. Verified lifted by restoring that wording and confirming no Error appears. The phrasing stays unknown command because it's already documented and released, not because it has to be — the comment in assert/core.sh now says so.

✅ Verification

2 new acceptance tests (the false positive, and a regression guard that a real shell error is still reported), RED first. make sa · make lint · bash build.sh bin -v → ✅ Build verified ✅ · 1708 sequential / 1667 parallel-simple-strict.