fix: type-safe stores + Any unbox/eq; finish multi-return deferrals

Type-checking gaps (segfault/corruption → compile errors):

- 0197: reject a store into an annotated slot whose value has no modeled
  coercion AND a different byte width (a 16-byte string into a 4-byte i32
  overran the slot and segfaulted). New checkAssignable / noneReinterpretIsUnsafe
  (coerce.zig, width via the LLVM-accurate typeSizeBytes) wired into every store
  site: var/const-decl, single + multi assignment (identifier/field/index/
  element/deref), named-return defaults. Same-width reinterpretations (*T→[*]T,
  i64→isize, fn-ref) and explicit xx/cast stay allowed; cascades suppressed via
  externalErrorsExist. Examples 1205, 1206.
- 0198: an implicit `Any → T` unbox is now a compile error (it blindly
  reinterpreted the boxed payload — silent garbage for a wrong scalar, a segfault
  for an aggregate). xx and compiler-generated match/pack unboxes are unaffected.
  Example 1207.
- 0199: `Any == <concrete>` (one operand Any) aborted the LLVM verifier — the
  comparison arm now fires when either operand is Any, boxing the concrete side
  first. Example 0654.

Multi-return deferrals (PLAN-MULTIRET #6 + named-order + D3 + generic):

- Reorder named return elements by name instead of requiring slot order; error on
  unknown/duplicate/missing (value-only AND full-failable-tuple forms). Examples
  0210, 0214.
- Reject a bare-paren (A, B) multi-return signature in generic-arg position
  (return-position-only). Example 0215.
- Multi-return closure types / lambda literals work via the reused tuple
  machinery (destructure, single-bind+field, lambda arg). Example 0216.
- Generic multi-return: positional works (0217); 0200: the named-slot
  implicit-return form now works for generic free fns + struct methods —
  monomorphizeFunction now calls bindNamedReturnSlots. Example 0218.

readme.md documents the annotated-store coercion rule; CHECKPOINT-MULTIRET.md
updated. Full corpus green (850/0).

examples/diagnostics/expected/1206-diagnostics-store-width-mismatch.stderr

@@ -0,0 +1,23 @@
+error: cannot assign 'a' of type 'i32' with a value of type 'string'
+  --> examples/diagnostics/1206-diagnostics-store-width-mismatch.sx:21:11
+   |
+21 |     s.a = "field";          // error: struct field, string ↛ i32
+   |           ^^^^^^^
+
+error: cannot assign 'element' of type 'i64' with a value of type 'string'
+  --> examples/diagnostics/1206-diagnostics-store-width-mismatch.sx:24:14
+   |
+24 |     arr[0] = "elem";        // error: array element, string ↛ i32
+   |              ^^^^^^
+
+error: cannot assign 'target' of type 'i32' with a value of type 'string'
+  --> examples/diagnostics/1206-diagnostics-store-width-mismatch.sx:28:11
+   |
+28 |     p.* = "deref";          // error: pointer deref, string ↛ i32
+   |           ^^^^^^^
+
+error: cannot assign 'u' of type 'i32' with a value of type 'string'
+  --> examples/diagnostics/1206-diagnostics-store-width-mismatch.sx:31:12
+   |
+31 |     u, v = "multi", 9;      // error: multi-assign target, string ↛ i32
+   |            ^^^^^^^