bugfix

    // Depth of unclosed top-level '{' brace groups -- a '|' inside one

    // (e.g. "echo hi | tr a-z A-Z" nested inside "{ ...; } > file" used as

    // one whole pipeline stage) is that group's OWN internal pipe, not a

    // boundary between two stages of THIS pipeline; only extractBraceGroup

    // Body(), later, ever needs to see it. Mirrors the existing "$(...)"/

    // backtick protection just below, which the SAME '|' check already

    // respects.

    // boundary between two stages of THIS pipeline; only

    // extractBraceGroupBody(), later, ever needs to see it. Mirrors the

    // existing "$(...)"/backtick protection just below, which the SAME

    // '|' check already respects.

    int braceDepth=0;

    auto flushStage=[&](){