refactor(joinir): add hints for Phase107/104/100 policy rejects

- balanced_depth_scan: missing_tail_inc, missing_return_i hints
- read_digits: missing_eos_guard, digit_set_mismatch hints
- pinned: missing_host_id hint
- Gradual migration (representative cases only)

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

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

src/mir/builder/control_flow/joinir/patterns/pattern2_inputs_facts_box.rs

@@ -193,9 +193,11 @@ impl Pattern2InputsFactsBox {
                             }
                             captured_env.insert_pinned(pinned_name, host_id);
                         } else {
-                            return Err(format!(
-                                "Pinned local '{}' not found in variable_map (internal error)",
-                                pinned_name
+                            use crate::mir::join_ir::lowering::error_tags;
+                            return Err(error_tags::freeze_with_hint(
+                                "phase100/pinned/missing_host_id",
+                                &format!("Pinned local '{}' not found in variable_map", pinned_name),
+                                "define the local before the loop (dominates loop entry)",
                             ));
                         }
                     }

src/mir/builder/control_flow/joinir/patterns/policies/balanced_depth_scan_policy.rs

@@ -176,16 +176,20 @@ fn extract_depth_scan_shape(
     }
 
     if !has_return_i {
-        return Err(error_tags::freeze(
-            "[phase107/balanced_depth_scan/contract/missing_return_i] missing `if depth == 0 { return i }` inside close branch",
+        return Err(error_tags::freeze_with_hint(
+            "phase107/balanced_depth_scan/missing_return_i",
+            "missing `if depth == 0 { return i }` inside close branch",
+            "ensure 'if depth == 0 { return i }' is inside close-branch",
         ));
     }
 
     // Require a tail `i = i + 1` at top-level (keeps the family narrow).
     let has_tail_inc = body.iter().any(|n| is_inc_assign(n, loop_counter_name, 1));
     if !has_tail_inc {
-        return Err(error_tags::freeze(
-            "[phase107/balanced_depth_scan/contract/missing_tail_inc] missing `i = i + 1` tail update",
+        return Err(error_tags::freeze_with_hint(
+            "phase107/balanced_depth_scan/missing_tail_inc",
+            "missing `i = i + 1` tail update",
+            "add tail update 'i = i + 1' at top-level",
         ));
     }
 
@@ -210,11 +214,15 @@ fn extract_depth_scan_shape(
 
 fn scan_control_flow(node: &ASTNode, return_count: &mut usize) -> Result<(), String> {
     match node {
-        ASTNode::Break { .. } => Err(error_tags::freeze(
-            "[phase107/balanced_depth_scan/contract/unexpected_break] break is not allowed in this family (return-in-loop only)",
+        ASTNode::Break { .. } => Err(error_tags::freeze_with_hint(
+            "phase107/balanced_depth_scan/unexpected_break",
+            "break is not allowed in this family (return-in-loop only)",
+            "use 'return i' form (no break) in this family",
         )),
-        ASTNode::Continue { .. } => Err(error_tags::freeze(
-            "[phase107/balanced_depth_scan/contract/unexpected_continue] continue is not allowed in this family",
+        ASTNode::Continue { .. } => Err(error_tags::freeze_with_hint(
+            "phase107/balanced_depth_scan/unexpected_continue",
+            "continue is not allowed in this family",
+            "remove continue, use tail increment instead",
         )),
         ASTNode::Return { .. } => {
             *return_count += 1;

src/mir/builder/control_flow/joinir/patterns/read_digits_break_condition_box.rs

@@ -6,6 +6,7 @@
 //!   - the digit literal set used in the final `if <is_digit> { ... } else { break }`
 
 use crate::ast::{ASTNode, BinaryOperator, LiteralValue};
+use crate::mir::join_ir::lowering::error_tags;
 
 pub(crate) struct ReadDigitsBreakConditionBox;
 
@@ -33,7 +34,11 @@ impl ReadDigitsBreakConditionBox {
         }
 
         let (ch_var, eos_cond) = find_eos_break_condition(body).ok_or_else(|| {
-            "[phase104/read-digits] missing `if ch == \"\" { break }` guard".to_string()
+            error_tags::freeze_with_hint(
+                "phase104/read_digits/missing_eos_guard",
+                "missing `if ch == \"\" { break }` guard",
+                "add 'if ch == \"\" { break }' before digit check",
+            )
         })?;
 
         let mut digit_literals: Vec<String> = Vec::new();
@@ -60,9 +65,10 @@ impl ReadDigitsBreakConditionBox {
         // Phase 104 minimal: require the canonical digit set.
         let expected: Vec<String> = (0..=9).map(|d| d.to_string()).collect();
         if digit_literals != expected {
-            return Err(format!(
-                "[phase104/read-digits] digit condition literal set mismatch: got={:?}, expected={:?}",
-                digit_literals, expected
+            return Err(error_tags::freeze_with_hint(
+                "phase104/read_digits/digit_set_mismatch",
+                &format!("digit condition literal set mismatch: got={:?}, expected={:?}", digit_literals, expected),
+                "use explicit 'ch == \"0\" || ... || ch == \"9\"'",
             ));
         }