fix(zig): emit constructorSlots from stack lowering placeholder ops

The Zig compiler's stack lowering was emitting push_data("") for
constructor parameter reads instead of proper placeholder ops. This
meant emitArtifact never recorded constructorSlots in the JSON, so
the SDK couldn't splice constructor args (PubKey, Addr, etc.) into
the locking script at deployment.

Now lowerPropertyRead emits a .placeholder instruction (with the
correct param_index) when a readonly property without an initial
value is referenced — matching the TS reference compiler's behavior.

Also added placeholder handling to StackInstruction, emitStackInstruction,
and the peephole optimizer's isPush/instEql helpers.

P2PKH, Escrow, and all other stateless contracts now deploy and
call correctly. 35/36 integration tests pass.

Author: icellan

compilers/zig/src/codegen/emit.zig

@@ -230,6 +230,10 @@ pub fn emitStackInstruction(ctx: *EmitContext, inst: types.StackInstruction) !vo
                 0;
             try ctx.emitScriptNumber(idx);
         },
+        .placeholder => |ph| {
+            try ctx.recordConstructorSlot(ph.param_index);
+            try ctx.emitOpcode(.op_0);
+        },
     }
 }
 

compilers/zig/src/ir/types.zig

@@ -339,7 +339,7 @@ pub const StackIf = struct { then: []StackOp, @"else": ?[]StackOp = null };
 pub const Placeholder = struct { param_index: u32, param_name: []const u8 };
 pub const PushValue = union(enum) { bytes: []const u8, integer: i64, boolean: bool };
 
-pub const StackInstruction = union(enum) { op: Opcode, push_data: []const u8, push_int: i64, push_bool: bool, push_codesep_index: void };
+pub const StackInstruction = union(enum) { op: Opcode, push_data: []const u8, push_int: i64, push_bool: bool, push_codesep_index: void, placeholder: Placeholder };
 
 // ============================================================================
 // Layer 4: Artifact Types (output of Pass 6: Emit) — maps to artifact.ts

compilers/zig/src/passes/peephole.zig

@@ -20,7 +20,7 @@ const max_iterations = 100;
 /// Returns true if the instruction is any push variant (push_int, push_data, push_bool).
 fn isPush(inst: Inst) bool {
     return switch (inst) {
-        .push_int, .push_data, .push_bool, .push_codesep_index => true,
+        .push_int, .push_data, .push_bool, .push_codesep_index, .placeholder => true,
         .op => false,
     };
 }
@@ -61,6 +61,7 @@ fn instEql(a: Inst, b: Inst) bool {
         .push_bool => |ba| ba == b.push_bool,
         .push_data => |da| std.mem.eql(u8, da, b.push_data),
         .push_codesep_index => true,
+        .placeholder => |pa| pa.param_index == b.placeholder.param_index,
     };
 }
 

compilers/zig/src/passes/stack_lower.zig

@@ -938,8 +938,14 @@ const LowerCtx = struct {
                 }
             }
         }
-        // Not found — push placeholder
-        try self.emitPushData("");
+        // Not found — push constructor param placeholder
+        // Determine param_index: count readonly properties before this one
+        var param_idx: u32 = 0;
+        for (self.program.properties) |prop| {
+            if (std.mem.eql(u8, prop.name, prop_name)) break;
+            if (prop.readonly and prop.initial_value == null) param_idx += 1;
+        }
+        try self.emit(.{ .placeholder = .{ .param_index = param_idx, .param_name = prop_name } });
         try self.stack.push(self.allocator, bind_name);
         self.trackDepth();
     }