fix: diagnose non-conforming protocol erasure instead of unreachable-thunk SIGABRT (issue 0176)

Erasing a type to a protocol when it conforms only via a free function
(not an explicit impl P for T) built a vtable of unreachable thunks ->
SIGABRT on first dispatch, with no diagnostic. Per specs.md erasure is
impl-driven, not structural, so the erasure was never valid.

Add a conformance gate (firstUnimplementedMethod in buildProtocolValue,
src/ir/lower/protocol.zig): emit a located diagnostic when a protocol
method has no reachable impl, or when an impl method introduces its own
type params (signature mismatch — it bails lazyLowerFunction and would
reach the unreachable thunk). A std.debug.panic tripwire guards the
diagnostics==null path so a non-conforming erasure can never silently
ship as undef. Gate<->thunk equivalence verified bidirectional.

Regressions: protocols/0419 (positive struct-field dispatch),
diagnostics/1197 (no-impl) + 1198 (generic-method signature mismatch).
Updated memory/0808 (it erased a non-conforming type that never
dispatched). Verified by 3+1 adversarial reviews, suite 788/0. Filed
adjacent bug 0178 (protocol impl method type-mismatch silent miscompile).

examples/memory/0808-memory-xx-value-routes-through-context-allocator.sx

@@ -31,13 +31,14 @@ impl Allocator for Tracer {
     }
 }
 
-ByValue :: struct { x: i64; y: i64; }
-
 main :: () -> i32 {
     tracer := Tracer.init();
     push Context.{ allocator = xx tracer, data = null } {
         // Struct-literal operand: rvalue → heap-copy through context.allocator.
-        ignore : Allocator = xx ByValue.{ x = 1, y = 2 };
+        // The erased type must actually `impl Allocator` (erasure is
+        // impl-driven, issue 0176), so use a fresh `Tracer.{}` rvalue — the
+        // copy is what we want routed through the active allocator.
+        ignore : Allocator = xx Tracer.{ count = 0 };
         _ = ignore;
     }
     print("Tracer.count = {}\n", tracer.count);