fix: validate protocol impl method signatures vs the protocol declaration (issue 0178)

The issue-0176 conformance gate was name-only, so an impl P for T with a
mismatched return/param type (or arity) built a wrong-ABI thunk that
silently miscompiled (exit 0, wrong value). firstUnimplementedMethod now
validates arity (after self), each param type, and the return type
against the protocol declaration, substituting protocol Self->concrete
via resolveProtoTypeSubSelf (recurses through pointer/many-pointer/
optional/slice/array so []Self<->[]T match; conservative .unresolved for
Self-in-generic-arg). Comparison is by structural formatTypeName
(alias/module/spelling independent); typesClearlyDiffer skips when either
side has an unresolved leaf at any depth, biasing against false-positives.

Regressions: diagnostics/1201 (negative), protocols/0420 (positive,
[]Self param). Verified by 3+3 adversarial reviews (a mid-fix []Self
false-positive was found and closed); suite 792/0.

examples/diagnostics/1201-diagnostics-protocol-impl-signature-mismatch.sx

@@ -0,0 +1,22 @@
+// A protocol-method impl with the right NAME but a mismatched RETURN type (or
+// parameter type) is a hard error, not a silent miscompile. Regression (issue
+// 0178): the issue-0176 conformance gate is name-based, so an `impl P for T`
+// whose method returns `bool` where the protocol method returns `i64` passed
+// the gate and built a wrong-ABI thunk — dispatch through the erased protocol
+// silently returned the wrong value (exit 0, no diagnostic). The conformance
+// gate now validates each impl method's signature (arity after `self`, each
+// parameter type, and the return type) against the protocol declaration, with
+// `Self` substituted to the concrete type (so the REQUIRED `*T` / `T` impl form
+// is NOT flagged). A clear type mismatch reports at the erasure site, exit 1.
+
+#import "modules/std.sx";
+
+P :: protocol { val :: (self: *Self) -> i64; }
+T :: struct { n: i64 = 7; }
+impl P for T { val :: (self: *T) -> bool { return true; } }   // return bool != i64
+
+main :: () {
+  t := T.{ n = 7 };
+  p : P = t;                  // <- 'T' does not implement 'P' (return type)
+  print("{}\n", p.val());
+}

examples/diagnostics/expected/1201-diagnostics-protocol-impl-signature-mismatch.exit

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

examples/diagnostics/expected/1201-diagnostics-protocol-impl-signature-mismatch.stderr

@@ -0,0 +1,5 @@
+error: 'T' does not implement protocol 'P': method 'val' has a mismatched signature — return type: protocol declares 'i64', impl declares 'bool' (a protocol-method impl must match the protocol's declared types exactly, with `Self` written as `T`)
+  --> examples/diagnostics/1201-diagnostics-protocol-impl-signature-mismatch.sx:20:3
+   |
+20 |   p : P = t;                  // <- 'T' does not implement 'P' (return type)
+   |   ^^^^^^^^^^

examples/diagnostics/expected/1201-diagnostics-protocol-impl-signature-mismatch.stdout

@@ -0,0 +1 @@
+