feat(interpreter): child shells count SHLVL like bash - #2622

Shell diagnostics now read like non-interactive bash:
`SCRIPT: line 2: x: command not found`, `bash: line 1: type: h: not found`
under -c, instead of a bare `bash: `. The name is BASH_SOURCE[0] when a
file is being read (a sourced file names itself), else $0.

- Interpreter messages go through Interpreter::diag_prefix / diag.
- Bundled builtins keep their `bash: name:` / `name:` text; the builtin
  dispatch point rewrites the line start before redirects or streaming
  see it. Usage lines, coreutils-style builtins and execution plans are
  untouched.
- eval syntax errors read `$0: eval: line N:`; failglob, arithmetic and
  write-error messages honor $0.
- `source missing` ends with a newline and reports the missing path;
  `source` with no operand exits 2 with bash's usage line.
- Redirect failures on while/for/if/case report the closing line.
- `set -u` names a bare positional `$2` like bash.
- Child shells (bash -c, scripts by path) get their own name and lines.
- Bash::set_interactive (used by the CLI REPL and Terminal) keeps the
  `bash: msg` form of `bash -i`.

bash-oracle: match 92 -> 98, strict 71 -> 89 of 106 (with #2622) (floors raised).

Shell diagnostics now read like non-interactive bash:
`SCRIPT: line 2: x: command not found`, `bash: line 1: type: h: not found`
under -c, instead of a bare `bash: `. The name is BASH_SOURCE[0] when a
file is being read (a sourced file names itself), else $0.

- Interpreter messages go through Interpreter::diag_prefix / diag.
- Bundled builtins keep their `bash: name:` / `name:` text; the builtin
  dispatch point rewrites the line start before redirects or streaming
  see it. Usage lines, coreutils-style builtins and execution plans are
  untouched.
- eval syntax errors read `$0: eval: line N:`; failglob, arithmetic and
  write-error messages honor $0.
- `source missing` ends with a newline and reports the missing path;
  `source` with no operand exits 2 with bash's usage line.
- Redirect failures on while/for/if/case report the closing line.
- `set -u` names a bare positional `$2` like bash.
- Child shells (bash -c, scripts by path) get their own name and lines.
- Bash::set_interactive (used by the CLI REPL and Terminal) keeps the
  `bash: msg` form of `bash -i`.

bash-oracle: match 92 -> 98, strict 71 -> 89 of 106 (with #2622) (floors raised).

Shell diagnostics now read like non-interactive bash:
`SCRIPT: line 2: x: command not found`, `bash: line 1: type: h: not found`
under -c, instead of a bare `bash: `. The name is BASH_SOURCE[0] when a
file is being read (a sourced file names itself), else $0.

- Interpreter messages go through Interpreter::diag_prefix / diag.
- Bundled builtins keep their `bash: name:` / `name:` text; the builtin
  dispatch point rewrites the line start before redirects or streaming
  see it. Usage lines, coreutils-style builtins and execution plans are
  untouched.
- eval syntax errors read `$0: eval: line N:`; failglob, arithmetic and
  write-error messages honor $0.
- `source missing` ends with a newline and reports the missing path;
  `source` with no operand exits 2 with bash's usage line.
- Redirect failures on while/for/if/case report the closing line.
- `set -u` names a bare positional `$2` like bash.
- Child shells (bash -c, scripts by path) get their own name and lines.
- Bash::set_interactive (used by the CLI REPL and Terminal) keeps the
  `bash: msg` form of `bash -i`.

bash-oracle: match 92 -> 98, strict 71 -> 89 of 106 (with #2622) (floors raised).