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.

lang/src/shared/json/utils/json_frag.hako

@@ -6,6 +6,108 @@ using selfhost.shared.json.core.json_cursor as JsonCursorBox
 using selfhost.shared.common.string_helpers as StringHelpers
 
 static box JsonFragBox {
+  // Toggle: enable Unicode \uXXXX decode in string readers
+  _decode_unicode_on() {
+    local v = env.get("HAKO_PARSER_DECODE_UNICODE")
+    if v == null { return 0 }
+    if v == "1" || v == "true" || v == "on" { return 1 }
+    return 0
+  }
+
+  // Decode simple escapes (\\ \" \/ \b \f \n \r \t) and \uXXXX (printable ASCII only)
+  _decode_escapes(s) {
+    if s == null { return null }
+    // Normalize common JSON double-escape: "\\uXXXX" -> "\uXXXX"
+    local src0 = "" + s
+    local n0 = src0.length()
+    local tmp = ""
+    local p = 0
+    loop(p < n0) {
+      local ch0 = src0.substring(p, p+1)
+      if ch0 == "\\" && p + 2 <= n0 && src0.substring(p+1, p+2) == "\\" {
+        if p + 3 <= n0 && src0.substring(p+2, p+3) == "u" {
+          tmp = tmp + "\\u"
+          p = p + 3
+          continue
+        }
+        // generic \\ -> \
+        tmp = tmp + "\\"
+        p = p + 2
+        continue
+      }
+      tmp = tmp + ch0
+      p = p + 1
+    }
+    local s1 = tmp
+    local n = s1.length()
+    if n == 0 { return s1 }
+    local out = ""
+    local i = 0
+    // Printable ASCII table for 0x20..0x7E
+    local ascii = " !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~"
+    loop(i < n) {
+      local ch = s1.substring(i, i+1)
+      if ch != "\\" { out = out + ch  i = i + 1  continue }
+      // escape
+      if i + 1 >= n { out = out + "\\"  i = i + 1  continue }
+      local e = s1.substring(i+1, i+2)
+      if e == "\\" { out = out + "\\"  i = i + 2  continue }
+      if e == "\"" { out = out + "\""  i = i + 2  continue }
+      if e == "/" { out = out + "/"  i = i + 2  continue }
+      if e == "b" { out = out + "\b"  i = i + 2  continue }
+      if e == "f" { out = out + "\f"  i = i + 2  continue }
+      if e == "n" { out = out + "\n"  i = i + 2  continue }
+      if e == "r" { out = out + "\r"  i = i + 2  continue }
+      if e == "t" { out = out + "\t"  i = i + 2  continue }
+      if e == "u" {
+        // \uXXXX (hex)
+        if i + 6 <= n {
+          local h = s1.substring(i+2, i+6)
+          // parse hex (limited)
+          local val = 0
+          local k = 0
+          local ok = 1
+          loop(k < 4) {
+            local c = h.substring(k, k+1)
+            local d = -1
+            if c >= "0" && c <= "9" { d = "0123456789".indexOf(c) }
+            else {
+              if c >= "a" && c <= "f" { d = 10 + ("abcdef".indexOf(c)) }
+              else { if c >= "A" && c <= "F" { d = 10 + ("ABCDEF".indexOf(c)) } else { ok = 0 }}
+            }
+            if d < 0 { ok = 0  break }
+            val = val * 16 + d
+            k = k + 1
+          }
+          if ok == 1 {
+            // Printable ASCII only (0x20..0x7E)
+            if val >= 32 && val <= 126 {
+              local pos = val - 32
+              out = out + ascii.substring(pos, pos+1)
+              i = i + 6
+              continue
+            }
+            // Surrogate pair handling: collapse \uD83D\uDE00 etc. into a single placeholder
+            // High surrogate range: 55296..56319
+            if val >= 55296 && val <= 56319 {
+              // skip following low surrogate if present
+              if i + 12 <= n && s1.substring(i+6, i+8) == "\\u" { i = i + 12 } else { i = i + 6 }
+              out = out + "?"
+              continue
+            }
+            // Non-ASCII BMP → placeholder
+            out = out + "?"
+            i = i + 6
+            continue
+          }
+        }
+      }
+      // Fallback: keep as-is for unknown escape
+      out = out + ch
+      i = i + 1
+    }
+    return out
+  }
   // 基本ヘルパ - VM fallback implementations for cross-box static calls
   index_of_from(hay, needle, pos) {
     // VM fallback: implement using substring + indexOf
@@ -62,7 +164,9 @@ static box JsonFragBox {
     local j = i
     loop(j < n) { if s.substring(j,j+1) == "\"" { break } j = j + 1 }
     if j <= i { return null }
-    return s.substring(i, j)
+    local raw = s.substring(i, j)
+    if me._decode_unicode_on() == 1 { return me._decode_escapes(raw) }
+    return raw
   }
   read_float_from(text, pos) {
     if text == null { return null }
@@ -154,7 +258,11 @@ static box JsonFragBox {
     if p >= 0 {
       local vstart = p + pat.length()              // start of value (right after opening quote)
       local vend = me._scan_string_end(seg, vstart - 1)
-      if vend > vstart { return seg.substring(vstart, vend) }
+      if vend > vstart {
+        local raw = seg.substring(vstart, vend)
+        if me._decode_unicode_on() == 1 { return me._decode_escapes(raw) }
+        return raw
+      }
     }
     return ""
   }