feat(mir): Phase 136 Step 2/7 - CoreContext extraction complete

Extract core ID generation fields from MirBuilder to improve organization:
- value_gen: ValueIdGenerator
- block_gen: BasicBlockIdGenerator
- next_binding_id: u32 (Phase 74)
- temp_slot_counter: u32
- debug_join_counter: u32

Implementation:
- Created src/mir/builder/core_context.rs (215 lines)
- Added core_ctx field to MirBuilder as SSOT
- Deprecated legacy fields with backward compat
- ID allocation methods use core_ctx and sync legacy fields
- Added next_block_id() helper, replaced 30+ block_gen.next() calls

Testing:
- cargo build --release: SUCCESS (193 warnings expected)
- cargo test --release --lib: 1004/1004 PASS (+7 tests)
- phase135_trim_mir_verify.sh: PASS
- phase132_exit_phi_parity.sh: 3/3 PASS

Progress: 2/7 Context extractions complete (TypeContext + CoreContext)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/builder/exprs_peek.rs

@@ -13,12 +13,12 @@ impl super::MirBuilder {
         let scr_val = self.build_expression_impl(scrutinee)?;
 
         // Prepare merge and result
-        let merge_block: BasicBlockId = self.block_gen.next();
+        let merge_block: BasicBlockId = self.next_block_id();
         let result_val = self.next_value_id();
         let mut phi_inputs: Vec<(BasicBlockId, ValueId)> = Vec::new();
 
         // Create dispatch block where we start comparing arms
-        let dispatch_block = self.block_gen.next();
+        let dispatch_block = self.next_block_id();
         // Jump from current block to dispatch (ensure terminator exists)
         let need_jump = {
             let cur = self.current_block;
@@ -39,7 +39,7 @@ impl super::MirBuilder {
 
         // If there are no arms, fall through to else directly
         if arms.is_empty() {
-            let else_block = self.block_gen.next();
+            let else_block = self.next_block_id();
             crate::mir::builder::emission::branch::emit_jump(self, else_block)?;
             self.start_new_block(else_block)?;
             let else_val = self.build_expression_impl(else_expr)?;
@@ -52,15 +52,15 @@ impl super::MirBuilder {
         }
 
         // Else block to handle default case
-        let else_block = self.block_gen.next();
+        let else_block = self.next_block_id();
 
         // Chain dispatch blocks for each arm
         let mut cur_dispatch = dispatch_block;
         for (i, (label, arm_expr)) in arms.iter().cloned().enumerate() {
-            let then_block = self.block_gen.next();
+            let then_block = self.next_block_id();
             // Next dispatch (only for non-last arm)
             let next_dispatch = if i + 1 < arms.len() {
-                Some(self.block_gen.next())
+                Some(self.next_block_id())
             } else {
                 None
             };