feat(runtime): add SIGSEGV signal handler to convert nil pointer dereference to recoverable panic

- Add z_rt_default.go with signal handler for SIGSEGV on non-wasm platforms
- Convert segmentation faults from nil pointer access to Go panic
- Enable recover() to catch nil pointer dereference errors
- Use build tag (!wasm) to maintain wasm platform compatibility
- Remove commented-out signal handling code from z_rt.go

This aligns llgo's behavior with standard Go, where accessing nil pointer
fields triggers a recoverable panic instead of immediate program crash.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

runtime/internal/runtime/z_rt.go

@@ -109,18 +109,6 @@ const MaxZero = 1024
 
 var ZeroVal [MaxZero]byte
 
-// func init() {
-// 	signal.Signal(c.Int(syscall.SIGSEGV), func(v c.Int) {
-// 		switch syscall.Signal(v) {
-// 		case syscall.SIGSEGV:
-// 			panic(errorString("invalid memory address or nil pointer dereference"))
-// 		default:
-// 			var buf [20]byte
-// 			panic(errorString("unexpected signal value: " + string(itoa(buf[:], uint64(v)))))
-// 		}
-// 	})
-// }
-
 // -----------------------------------------------------------------------------
 
 type SigjmpBuf struct {

runtime/internal/runtime/z_signal.go

@@ -0,0 +1,48 @@
+//go:build !wasm && !baremetal
+
+/*
+ * Copyright (c) 2024 The GoPlus Authors (goplus.org). All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package runtime
+
+import (
+	c "github.com/goplus/llgo/runtime/internal/clite"
+	"github.com/goplus/llgo/runtime/internal/clite/signal"
+)
+
+const (
+	// SIGSEGV is signal number 11 on all Unix-like systems (Linux, Darwin, BSD, etc.)
+	// Using a hardcoded constant avoids importing the syscall package, which would
+	// introduce dependencies on errors and internal/reflectlite packages that cause
+	// linking issues in c-shared and c-archive build modes.
+	SIGSEGV = c.Int(0xb)
+)
+
+// This file contains platform-specific runtime initialization for non-wasm targets.
+// The SIGSEGV signal handler enables Go-style panic recovery for nil pointer dereferences
+// instead of immediate process termination.
+//
+// For wasm platform compatibility, signal handling is excluded via build tags.
+// See PR #1059 for wasm platform requirements.
+func init() {
+	signal.Signal(SIGSEGV, func(v c.Int) {
+		if v == SIGSEGV {
+			panic(errorString("invalid memory address or nil pointer dereference"))
+		}
+		var buf [20]byte
+		panic(errorString("unexpected signal value: " + string(itoa(buf[:], uint64(v)))))
+	})
+}