feat(asm): encode indirect JMP and CALL on all four architectures

Assisted-by: GLM 5.3 Flash

docs/ARCHITECTURE.md

@@ -161,7 +161,8 @@ Two deeper analyses sit on top of the AST:
 - **`unreachable-code`.**  Code after a `RET` and before the next label is
   dead.  The check is suppressed for any function whose reachability cannot be
   decided statically: those using PC-relative jumps (`JMP 2(PC)`),
-  register-indirect branches (`JALR`/`JR`/`JIRL`/`BR`/`BLR`), or living in a
+  register-indirect branches (`JALR`/`JR`/`JIRL`/`BR`/`BLR`, or a `JMP`/`CALL`
+  through a register or memory operand), or living in a
   file with `#ifdef` conditionals.  `UNDEF` is deliberately not a terminator:
   code after it is occasionally intentional metadata.
 - **`register-clobber` (register liveness).**  The linter builds the function's