fix: reject direct assignment to a tagged-union variant member

A tagged union (enum-with-payload) is laid out { tag, payload }, but a
direct member write `s.rect = payload` lowered to a payload-only store
(union_gep into field 1) with no tag store — the discriminant went stale,
so a later match/== took the wrong arm with no diagnostic (issue 0136).
The read path already distinguishes tagged unions (enum_payload/enum_tag);
the write path treated them like plain unions.

A variant is set via construction (`s = .variant(payload)`, which writes
both tag and payload). A direct member write can't safely set the tag (the
active variant isn't known at the write site), so it is now rejected with a
diagnostic pointing to construction. A new diagTaggedUnionVariantWrite guard
— reusing the shared fieldLvalueResolve matcher, applied at both store sites
(lowerAssignment, lowerMultiAssign) — fires only for a whole-variant write
on a tagged union. Plain `union` writes and nested sub-field writes
(`s.rect.w = ...`) are unaffected.

Resolves issue 0136. Tests: examples/0185 (rejected), 0186 (nested write +
construction still work). specs.md / readme.md updated.

examples/expected/0185-types-tagged-union-member-assign-rejected.exit

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

examples/expected/0185-types-tagged-union-member-assign-rejected.stderr

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+error: cannot assign to tagged-union variant 'rect' directly — a member write sets the payload but leaves the tag stale; construct the variant instead (e.g. `x = .rect(...)`)
+  --> examples/0185-types-tagged-union-member-assign-rejected.sx:18:5
+   |
+18 |     s.rect = .{ w = 4.0, h = 2.0 };   // rejected — use `s = .rect(.{...})` instead
+   |     ^^^^^^

examples/expected/0185-types-tagged-union-member-assign-rejected.stdout

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+

examples/expected/0186-types-tagged-union-nested-field-write.exit

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

examples/expected/0186-types-tagged-union-nested-field-write.stderr

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+

examples/expected/0186-types-tagged-union-nested-field-write.stdout

@@ -0,0 +1,2 @@
+w=9.000000 h=2.000000
+c=3.500000