The error slot of a value-carrying failable can no longer be silently dropped
on a bare destructure. In lowerDestructureDecl, when the RHS is failable
(errorChannelOf(ty) != null), the error slot (always the last tuple field)
must be bound to a non-`_` name. Reject when it is omitted entirely (fewer
names than slots — e.g. `a, c := inc(5)` for `inc: -> (s32,s32,!E)`) or bound
to `_` (`v, _ := parse(5)`).
The `try` / `catch` / `or value` consumer forms all strip the error channel
(their result type is non-failable), so the check never fires on them — only a
bare failable destructure is rejected. Value-slot `_` discards stay legal
(`a, _, ae := pair()` binds the error).
This is the discard-rejection slice of E1.8; the path-sensitive flow-check
(value live only where err==null is provable) is a separate follow-up.
examples/236-failable-discard-reject.sx covers both rejected shapes (exit 1).
Gates: zig build, zig build test, 274/274 examples.
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