108 reject compound raw slice return

// Pins that the raw-slice wall is STRUCTURAL, not textual. 510_104 rejects a
// bare `-> []const u8`; the same slice one layer in must be rejected for the
// same reason. `string` is the surface text type in every position, and a
// compound is still a surface type.
//
// The guarded shape is the wall's method rather than its verdict: the check is
// string equality against the literal "[]const u8", so anything that merely
// CONTAINS the slice — `[][]const u8`, and presumably `?[]const u8` and
// `*[]const u8` — walks past it and compiles clean to a binary. A wall matched
// by spelling is not a wall; it is a spell-checker.
//
// Deferred on purpose (Lars, 2026-07-25): the fix rides with the wider sweep
// over host syntax reaching the surface, not as a patch to one of three
// std.mem.eql sites. This pin holds the gap on the board until that sweep runs,
// so the deferral is visible rather than remembered.
~tor echo { msg: string } -> [][]const u8