MIR Codegen: Assembler Label & Jump Resolution

## Summary Implement correct label, jump, and PUSH-width resolution so scheduled EVM instruction sequences become final bytecode. **Parent issue:** #687 ## Context Currently, jumps in the codegen are placeholders. We need a proper assembler that: 1. Assigns symbolic labels to basic blocks 2. Computes concrete byte offsets 3. Handles variable-width PUSH instructions (the target offset affects the PUSH size, which affects all subsequent offsets) ## Tasks ### Label abstraction - [ ] Represent basic block labels as symbolic IDs - [ ] `JUMP`/`JUMPI` instructions target labels, not numeric offsets - [ ] Track label definitions and references ### Two-pass assembly - [ ] **Pass 1**: Linearize instructions, assign tentative offsets, track label positions - [ ] Compute sizes of variable-width instructions (`PUSHn`) - [ ] **Pass 2**: Re-run if PUSH widths changed due to offset size - [ ] Iterate until fixed-point (typically 2-3 iterations max) ### Jump resolution - [ ] Replace each label reference with concrete offset bytes - [ ] Emit correct `PUSHn` followed by `JUMP`/`JUMPI` - [ ] Handle forward and backward jumps ### JUMPDEST handling - [ ] Ensure every jump target has preceding `JUMPDEST` - [ ] Maintain mapping `label → JUMPDEST offset` - [ ] Insert `JUMPDEST` opcodes during linearization ## Example ``` ; Symbolic assembly PUSH label_else JUMPI PUSH 42 PUSH label_end JUMP label_else: JUMPDEST PUSH 0 label_end: JUMPDEST RETURN ; After resolution (assuming offsets) PUSH1 0x0A ; offset of label_else JUMPI PUSH1 0x2A ; 42 PUSH1 0x0C ; offset of label_end JUMP JUMPDEST ; 0x0A: label_else PUSH1 0x00 JUMPDEST ; 0x0C: label_end RETURN ``` ## Patterns to follow **From Venom:** - Structured assembler with labels and separate patching step **From Sonatina:** - Clean separation between IR and assembly, symbol resolution passes ## Acceptance Criteria - [ ] Assembly tests: branches, loops, function dispatch have correct JUMP destinations - [ ] Round-trip tests: assemble → disassemble → compare structure - [ ] No invalid jumps or missing JUMPDESTs under EVM verification - [ ] Variable-width PUSH correctly sized (PUSH1 vs PUSH2 vs PUSH3) ## Estimated Complexity **Small-Medium** - Well-understood algorithm but requires careful offset tracking ## Dependencies - MIR codegen skeleton (done) - #696 or #697 (Stack scheduling) for realistic instruction streams