Stream merge redirects#276
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New single-token merge redirects (no internal spaces): 2>&1 sends stderr to stdout's final destination, 1>&2 sends stdout to stderr's. Unlike POSIX these are not order-sensitive fd duplication; the merged stream aliases the other stream's resolved destination, so `2>&1 *` captures both streams interleaved and `[[make] 2>&1 [grep err]] |;` sends stderr through the pipe, cross-platform. Works on command lists, pipeline stages, and quotations. Each stream now has exactly one destination: combining two destinations on the same stream (capture + file redirect, &> + 2>, a second >, merge + anything on that stream, circular double merge, or 2>&1 with <>) is an error at both runtime and in the static type checker, with identical messages. Previously the extra redirect was silently ignored or last-wins. The checker tracks per-stream destination states (file, in-place, merged) on the command type; <> now statically requires a Path target to match the runtime. Also fixes: loop quotations ignored stderr redirects and append mode and dropped the enclosing quotation's redirected streams (both loop paths now build their context via BuildExecutionContext), and CopyListParams never copied AppendOutput. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Quotations accept only the redirects that don't change their stack
effect: file redirects (>, >>, 2>, 2>>, &>, &>>), stdin (<), and the
merges (2>&1, 1>&2). The captures (*, *b, ^, ^b) and the in-place
redirect (<>) are rejected on quotations with a clear message at both
runtime and in the static type checker.
Previously a capture on a quotation fell into the multiplication branch
at runtime ("Cannot do '*' operation on a stack with only one item"),
and the type checker wrongly accepted <> on a quotation that the
runtime then rejected. Capture individual command lists instead, e.g.
[[cmd1] [cmd2]] (* !) map.
Docs now state the allowed/excluded operator sets, that destination
conflicts on quotations are enforced at runtime (redirects never change
a quotation's stack effect, so they are invisible to the checker), and
the open-per-execution rule: a >-redirected quotation truncates its
file on every execution (use >> to accumulate), except loop, which
opens the file once for all iterations.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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