026 pattern branch name is metadata channel

// Test: A pattern-named branch's text is a comptime metadata channel
//
// A bracketed branch name is an opaque string — spaces, colons, all of
// it — that the emitter quotes into the union field name verbatim.
// A proc reflecting @This().Output reads the full text back, so the
// branch name alone can carry machine-facing and human-facing data:
// "tag: description" splits on the first ':' into a criteria key and
// a description, with nothing declared twice. Continuations match the
// pattern name by equality and bind the payload.

~import std/io

const std = @import("std");

~tor classify {}
| [bug: Bug reports] f64
| [feature: Feature requests] f64
| [other: Other messages] f64
| exhausted

fn tagOf(comptime name: []const u8) []const u8 {
    return if (std.mem.indexOfScalar(u8, name, ':')) |i| name[0..i] else name;
}
fn descOf(comptime name: []const u8) []const u8 {
    return if (std.mem.indexOfScalar(u8, name, ':')) |i| std.mem.trim(u8, name[i + 1 ..], " ") else name;
}

~proc classify|zig {
    var buf: [256]u8 = undefined;
    const fields = @typeInfo(@This().Output).@"union".fields;
    inline for (fields) |f| {
        const line = std.fmt.bufPrint(&buf, "{s}|{s}\n", .{ tagOf(f.name), descOf(f.name) }) catch unreachable;
        _ = std.posix.write(1, line) catch 0;
    }
    return @unionInit(@This().Output, "bug: Bug reports", 0.93);
}

~classify()
| [bug: Bug reports] p |> std/io:print.ln("bug fires p={{ p:f }}")
| [feature: Feature requests] p |> std/io:print.ln("feature fires")
| [other: Other messages] p |> std/io:print.ln("other fires")
| exhausted |> std/io:print.ln("exhausted")