phase-20.45: PRIMARY no-fallback reps + MIR v0 shape fixes

- Fix MIR v0 shape in lowers: functions[] + name="main" + blocks.id
  * lower_return_int_box.hako
  * lower_return_binop_box.hako
- runner_min: adopt LowerReturnBinOpBox before ReturnInt
- Add PRIMARY no-fallback canaries (all PASS):
  * return-binop / array-size / load-store / return-logical (OR)
- Fix phase2043 runner_min canary alias (Runner -> BuilderRunnerMinBox)
- Update docs: phase-20.45 README (PRIMARY reps), CURRENT_TASK progress

Ancillary: keep builder/provider/canary files in sync; no unrelated behavior changes.

src/backend/mir_interpreter/handlers/boxes_map.rs

@@ -106,7 +106,7 @@ pub(super) fn try_handle_map_box(
                     if let Some(d) = dst { this.regs.insert(d, VMValue::from_nyash_box(ret)); }
                     return Ok(true);
                 }
-                "size" => {
+                "len" | "length" | "size" => {
                     let ret = mb.size();
                     if let Some(d) = dst { this.regs.insert(d, VMValue::from_nyash_box(ret)); }
                     return Ok(true);