ERR/E1.8: reject dropping a failable's error slot on destructure

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.

tests/expected/236-failable-discard-reject.exit

@@ -0,0 +1 @@
+1

tests/expected/236-failable-discard-reject.txt

@@ -0,0 +1,11 @@
+error: the error slot of a failable cannot be dropped — bind it (`v, err := …`) and handle it, or use `try` / `catch`
+  --> /Users/agra/projects/sx/examples/236-failable-discard-reject.sx:27:13
+   |
+27 |     a, b := pair(5);     // ERROR: error slot omitted (3 slots, 2 names)
+   |             ^^^^^^^
+
+error: the error slot of a failable cannot be dropped — bind it (`v, err := …`) and handle it, or use `try` / `catch`
+  --> /Users/agra/projects/sx/examples/236-failable-discard-reject.sx:28:13
+   |
+28 |     v, _ := parse(5);    // ERROR: error slot discarded with `_`
+   |             ^^^^^^^^