Builder: reserve slots for user-defined instance methods (start at 4)\n- Deterministic per-type slot assignment via sorted names\n- Enables method_id emission for user boxes when type known\n- Docs updated

docs/development/roadmap/phases/phase-9/phase_9_79b_1_unified_registry_ids_and_builder_slotting.md

@@ -18,6 +18,7 @@ Last Updated: 2025-08-26
 - MIR Builder
   - When receiver type can be inferred, emit `BoxCall { method_id }` (slot ID) instead of name.
   - Add late-bind fallback path for unresolved sites (keeps current behavior).
+  - Reserve user-defined instance methods slots deterministically (start at 4; universal [0..3]).
 - Debug scaffolding
   - Add `MIRDebugInfo` container types (empty by default) for ID→name mapping (off by default).
 - Docs
@@ -46,4 +47,3 @@ Last Updated: 2025-08-26
 
 ## Roll-forward
 - Proceed to 9.79b.2 (VM vtable/thunk + mono-PIC).
-

src/mir/builder.rs

@@ -9,6 +9,7 @@ use super::{
     FunctionSignature, ValueId, ConstValue, BinaryOp, UnaryOp, CompareOp,
     MirType, EffectMask, Effect, BasicBlockIdGenerator, ValueIdGenerator
 };
+use super::slot_registry::{get_or_assign_type_id, reserve_method_slot};
 use super::slot_registry::resolve_slot_by_type_name;
 use crate::ast::{ASTNode, LiteralValue, BinaryOperator};
 use std::collections::HashMap;
@@ -1424,6 +1425,22 @@ impl MirBuilder {
             self.weak_fields_by_box.insert(name.clone(), set);
         }
         
+        // Reserve method slots for user-defined instance methods (deterministic, starts at 4)
+        let mut instance_methods: Vec<String> = Vec::new();
+        for (mname, mast) in &methods {
+            if let ASTNode::FunctionDeclaration { is_static, .. } = mast {
+                if !*is_static { instance_methods.push(mname.clone()); }
+            }
+        }
+        instance_methods.sort();
+        if !instance_methods.is_empty() {
+            let tyid = get_or_assign_type_id(&name);
+            for (i, m) in instance_methods.iter().enumerate() {
+                let slot = 4u16.saturating_add(i as u16);
+                reserve_method_slot(tyid, m, slot);
+            }
+        }
+        
         // Process methods - now methods is a HashMap
         for (method_name, method_ast) in methods {
             if let ASTNode::FunctionDeclaration { .. } = method_ast {