fix(interpreter): diagnostics carry bash's "$0: line N:" prefix

[chaliy]

Requested by Михайло · project thread

What changed

Shell diagnostics now carry bash's non-interactive prefix: the running script's name and line ($0: line N: ) instead of a bare bash: . This covers interpreter errors (command not found, arithmetic, redirects, readonly, set -u, failglob) and the shell builtins bash ships (cd, read, type, pushd, source, …). Coreutils-style builtins keep their own cat: missing: ... form, as on a real system.

• A sourced file names itself; bash -c children print bash: line N:.
• Bash::set_interactive(true) (used by the CLI REPL and Terminal) keeps the bash: msg form of bash -i.
• source missing reports SCRIPT: line N: missing: No such file or directory; source with no operand exits 2 with bash's usage line.
• Redirect failures on while/for/if/case ... done < f report the closing line.
• Under set -u, a bare positional is named $2, as bash does.

Why

Agents read stderr to locate a failure. Bash's script.sh: line 12: points at the line; bashkit's bash: did not. It was also the largest stderr gap on the bash-oracle scoreboard.

Before / After

Before, every one of these lines started with bash: and no line number. After:

$ bashkit t.sh
ok
t.sh: line 2: nosuch: command not found. Did you mean: touch?
t.sh: line 3: cd: /nope: No such file or directory
$ bash t.sh
ok
t.sh: line 2: nosuch: command not found
t.sh: line 3: cd: /nope: No such file or directory

The "Did you mean" hint is a deliberate sandbox feature and stays.

bash-oracle on 106 cases: match 92 → 98, strict 71 → 89 (one each from #2622 SHLVL, now on main) (floors raised in floor.txt).

Risk

Diagnostic text changes, so callers that matched bash: ... exactly will see the new prefix. Lines starting bash: are rewritten at the single builtin dispatch point; the decision is recorded at the top of interpreter/mod.rs. Known remaining gaps are in knowledge/operations/limitations.md ("Diagnostic prefix"): compound commands without spans report a stale line, and functions from sourced files name the caller's file.

Checklist

• Failing tests first: tests/integration/diagnostic_prefix_tests.rs (23 cases, expected text checked against bash 5.2)
• cargo fmt, clippy -D warnings, integration (1431), lib (2933), bashkit-cli, bash_comparison --ignored
• Knowledge: log.md, limitations.md, bash-oracle README

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