fix(debug): hardware watchpoints, signal stops and breakpoint restore

Assisted-by: GLM 5.3

debug/regs_linux_arm64.go

@@ -130,5 +130,11 @@ func (r *Regs) RegValue(name string) (uint64, bool) {
 // breakpointInsn is the software breakpoint instruction (BRK #0).
 var breakpointInsn = []byte{0x00, 0x00, 0x20, 0xD4} // BRK #0
 
-// breakpointPCAdjust is how far PC is past the breakpoint instruction after a trap.
-const breakpointPCAdjust = 4
+// breakpointPCAdjust is how far PC is past the breakpoint instruction after
+// a trap: 0, because the arm64 kernel delivers the BRK SIGTRAP with the PC
+// still on the BRK.  do_el0_brk64 calls send_user_sigtrap, which uses
+// instruction_pointer(regs) unmodified (arch/arm64/kernel/debug-monitors.c);
+// only the kernel-internal skip paths advance the PC.  GDB history agrees:
+// decr_pc_after_break on aarch64 Linux is 0 (the +4 variant was a QEMU bug,
+// sourceware PR 17280).
+const breakpointPCAdjust = 0