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/types/expected/0210-types-multi-return-name-order.exit

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

examples/types/expected/0210-types-multi-return-name-order.stderr

@@ -1,11 +1 @@
-error: named return element 'b' does not match the slot 'a' at position 0 — name the elements in slot order
-  --> examples/types/0210-types-multi-return-name-order.sx:6:5
-   |
- 6 |     return b = n, a = n + 1;   // out of slot order
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: named return element 'a' does not match the slot 'b' at position 1 — name the elements in slot order
-  --> examples/types/0210-types-multi-return-name-order.sx:6:5
-   |
- 6 |     return b = n, a = n + 1;   // out of slot order
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^

examples/types/expected/0210-types-multi-return-name-order.stdout

@@ -1 +1,2 @@
-
+6 5
+11 10

examples/types/expected/0214-types-multi-return-name-invalid.exit

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

examples/types/expected/0214-types-multi-return-name-invalid.stderr

@@ -0,0 +1,11 @@
+error: named return element 'c' does not name any return slot
+  --> examples/types/0214-types-multi-return-name-invalid.sx:7:5
+   |
+ 7 |     return a = n, c = n + 1;   // error: 'c' names no return slot
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: named return element 'a' is given more than once
+  --> examples/types/0214-types-multi-return-name-invalid.sx:10:5
+   |
+10 |     return a = n, a = n + 1;   // error: 'a' given more than once
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^

examples/types/expected/0214-types-multi-return-name-invalid.stdout

@@ -0,0 +1 @@
+

examples/types/expected/0215-types-multi-return-as-generic-arg.exit

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

examples/types/expected/0215-types-multi-return-as-generic-arg.stderr

@@ -0,0 +1,5 @@
+error: a bare-paren `(A, B)` is a multi-return signature, valid only as a return type; a tuple-valued generic type argument uses `Tuple(…)`
+  --> examples/types/0215-types-multi-return-as-generic-arg.sx:9:15
+   |
+ 9 |     xs : List((i32, bool)) = ---;   // error: multi-return signature, not a type
+   |               ^^^^^^^^^^^

examples/types/expected/0215-types-multi-return-as-generic-arg.stdout

@@ -0,0 +1 @@
+

examples/types/expected/0216-types-multi-return-closure.exit

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

examples/types/expected/0216-types-multi-return-closure.stderr

@@ -0,0 +1 @@
+

examples/types/expected/0216-types-multi-return-closure.stdout

@@ -0,0 +1,3 @@
+7 true
+7 true
+9

examples/types/expected/0217-types-multi-return-generic.exit

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

examples/types/expected/0217-types-multi-return-generic.stderr

@@ -0,0 +1 @@
+

examples/types/expected/0217-types-multi-return-generic.stdout

@@ -0,0 +1,3 @@
+7 true
+hi 42
+3 false

examples/types/expected/0218-types-multi-return-generic-named.exit

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

examples/types/expected/0218-types-multi-return-generic-named.stderr

@@ -0,0 +1 @@
+

examples/types/expected/0218-types-multi-return-generic-named.stdout

@@ -0,0 +1,4 @@
+7 true
+5 99
+3 false
+10 11