fix: diagnose ?(?T) tuple-payload mismatch instead of malformed IR (issue 0165)

In type position (T) is a 1-tuple (specs.md:843), so ?(?i64) is
optional(tuple(?i64)); assigning a bare ?i64 had coerceToType classify
.none and pass the value through, then optionalWrap built a corrupt
insertvalue that aborted the LLVM verifier. After coercing toward an
optional's child, verify the coerced type equals the child type
(stmt.zig decl-init + coerce.zig .optional_wrap); on mismatch emit a
located diagnostic (tuple-specific note only when the child is a tuple).
formatTypeName now renders tuples as (x: i64, y: i64).

Regressions: optionals/0911 (nested optional via alias, round-trip),
diagnostics/1195 (the mismatch diagnostic). Updated diagnostics/1101 +
protocols/0414 goldens for the improved tuple type-name rendering.
Verified by 3 adversarial reviews. Filed adjacent bug 0171 (?any child
not canonicalized).

examples/optionals/0911-nested-optional-via-alias.sx

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+// Nested optional `?(?i64)` written via a type alias — `Opt :: ?i64; ?Opt`.
+// The outer optional's payload is the inner optional, so the layout is the
+// well-formed double-wrap `{ {i64,i1}, i1 }`. Assigning an inner `?i64`,
+// unwrapping the outer to recover the inner `?i64`, and unwrapping that all
+// round-trip cleanly.
+//
+// Regression (issue 0165): the type-table interning always produced the
+// correct `{ {i64,i1}, i1 }` for a genuine nested optional — this locks that in.
+#import "modules/std.sx";
+
+Opt :: ?i64;
+
+main :: () {
+  inner : ?i64 = 5;
+  outer : ?Opt = inner;        // ?(?i64) — payload is the inner optional
+
+  print("outer present: {}\n", outer != null);
+
+  mid := outer!;               // unwrap outer  -> ?i64
+  print("mid present: {}\n", mid != null);
+  print("value: {}\n", mid!);  // unwrap inner  -> 5
+
+  // null-coalesce through the recovered inner optional
+  print("coalesced: {}\n", mid ?? 0);
+
+  // a none outer
+  empty : ?Opt = null;
+  print("empty present: {}\n", empty != null);
+}

examples/optionals/expected/0911-nested-optional-via-alias.exit

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+0

examples/optionals/expected/0911-nested-optional-via-alias.stderr

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+

examples/optionals/expected/0911-nested-optional-via-alias.stdout

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+outer present: true
+mid present: true
+value: 5
+coalesced: 5
+empty present: false