feat(debug): port the debugger to FreeBSD

Assisted-by: GLM 5.3 Flash

docs/ARCHITECTURE.md

@@ -579,7 +579,9 @@ AST, so neither depends on an encoding.
   --ground-truth`, `go list -json -export` locates the archives of the packages
   a GOOBJ object references, and `_gen` parses
   `$GOROOT/src/cmd/internal/obj/<arch>/anames.go` to rebuild the tables.
-- **Linux process interfaces** for the dynamic work: `mmap` and `mprotect` for
-  the JIT mapping, ptrace with `/proc/pid/mem` for the debugger.  That is why
+- **Linux and FreeBSD process interfaces** for the dynamic work: `mmap` and
+  `mprotect` for the JIT mapping, ptrace for the debugger — with tracee
+  memory through `/proc/pid/mem` on Linux and through `PT_IO` on FreeBSD,
+  and the tracee's stop reports read from `PT_LWPINFO` there.  That is why
   `verify` runs a JIT check only when the host architecture matches the
-  kernel's, and why `debug` is Linux-only.
+  kernel's, and why `debug` is bounded to those two kernels.

docs/CLI.md

@@ -287,7 +287,8 @@ Usage: gasm debug <file.s> --func <name>
 The debugger re-executes the binary it is running as (`os.Executable()`) for the
 traced child, so the child is the same `gasm`, whether it is installed on `$PATH`
 or run with `go run ./cmd/gasm`; nothing has to be installed first. Requires
-Linux (ptrace), and all four architectures are supported.
+Linux or FreeBSD (ptrace): all four architectures on Linux, amd64, arm64 and
+riscv64 on FreeBSD.
 
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