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/tests/tokenizer_unicode_toggle.rs

@@ -0,0 +1,33 @@
+use crate::tokenizer::{NyashTokenizer, TokenType};
+
+fn collect_string_token(src: &str) -> String {
+    let mut t = NyashTokenizer::new(src);
+    let tokens = t.tokenize().expect("tokenize");
+    // Expect first non-EOF token to be STRING
+    for tok in tokens {
+        if let TokenType::STRING(s) = tok.token_type { return s; }
+    }
+    panic!("no STRING token found");
+}
+
+#[test]
+fn unicode_decode_toggle_off_keeps_literal() {
+    // OFF by default
+    std::env::remove_var("NYASH_PARSER_DECODE_UNICODE");
+    std::env::remove_var("HAKO_PARSER_DECODE_UNICODE");
+    let s = collect_string_token("\"\\u0041\"");
+    assert_eq!(s, "\\u0041");
+}
+
+#[test]
+fn unicode_decode_toggle_on_decodes_basic_and_surrogate() {
+    // ON: enable decode
+    std::env::set_var("NYASH_PARSER_DECODE_UNICODE", "1");
+    let s = collect_string_token("\"\\u0041\"");
+    assert_eq!(s, "A");
+
+    let s2 = collect_string_token("\"\\uD83D\\uDE00\"");
+    // Expect surrogate pair to decode into one char (😀)
+    assert_eq!(s2.chars().count(), 1);
+}
+