feat: ALLOC_REMOTE reads fref+2 for data-driven ALLOC_SHARED

Task 1 Phase 4a Subcomponent A:

Updated ALLOC_REMOTE handler in emu/pe.py to read fref+2 from frame constants
and conditionally emit ALLOC_SHARED vs ALLOC based on whether parent_act_id
is non-zero.

Implementation details:
- Read fref+0 (target PE), fref+1 (target act_id), and new fref+2 (parent act_id)
- If parent_act (fref+2) is non-zero: emit FrameControlToken with op=ALLOC_SHARED and payload=parent_act
- If parent_act is zero/None: emit op=ALLOC and payload=0 (backwards compatible)
- Frame slots at fref+0, fref+1, fref+2 are assumed to be int values per codegen contract
- No runtime type checking added (consistent with existing ALLOC_REMOTE pattern)

Accepts AC4.1 (data-driven shared allocation) and AC4.2 (no new opcodes).

All 1290 tests pass. Backwards compatibility verified: existing tests with
only fref+0/fref+1 populated default fref+2 to None/0 (falsy), so they
emit op=ALLOC as before.

emu/pe.py

···
             yield self.env.timeout(1)  # EMIT cycle
             self._do_emit_new(inst, result, False, token.act_id, frame_id)
         elif inst.opcode == RoutingOp.ALLOC_REMOTE:
-            # PE-level: read target PE and act_id from frame constants
+            # PE-level: read target PE, act_id, and optional parent act_id from frame constants
+            # fref+0: target PE
+            # fref+1: target act_id
+            # fref+2: parent act_id (0 = fresh ALLOC, non-zero = ALLOC_SHARED)
             # Total: 4 cycles (dequeue + IFETCH + EXECUTE + EMIT)
             target_pe = self.frames[frame_id][inst.fref] if inst.fref < len(self.frames[frame_id]) else 0
             target_act = self.frames[frame_id][inst.fref + 1] if inst.fref + 1 < len(self.frames[frame_id]) else 0
+            parent_act = self.frames[frame_id][inst.fref + 2] if inst.fref + 2 < len(self.frames[frame_id]) else 0
+
+            if parent_act:
+                alloc_op = FrameOp.ALLOC_SHARED
+                payload = parent_act
+            else:
+                alloc_op = FrameOp.ALLOC
+                payload = 0
+
             fct = FrameControlToken(
                 target=target_pe,
                 act_id=target_act,
-                op=FrameOp.ALLOC,
-                payload=0,
+                op=alloc_op,
+                payload=payload,
             )
             self._on_event(Executed(
                 time=self.env.now, component=self._component,