fix(angular): use fail-fast error handling for invariant violations and remove panics (#37)

- Make i18n metadata type checks fail-fast (return early) instead of
  silently dropping placeholders, matching Angular's throw behavior
- Make unknown @for loop variable checks fail-fast instead of emitting
  empty expressions, matching Angular's AssertionError throw
- Add 4 unit tests for the fail-fast behavior
- Replace all panic!/unreachable!/debug_assert! in production code with
  diagnostics-style error reporting or safe fallbacks (10 instances)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: Brooooooklyn

crates/oxc_angular_compiler/src/i18n/ast.rs

@@ -575,9 +575,7 @@ impl Visitor for CloneVisitor {
             Node::IcuPlaceholder(IcuPlaceholder::new(icu, ph.name.clone(), ph.source_span.clone()))
         } else {
             // visit_icu should always return Node::Icu by design.
-            // This is a compiler bug if we reach here.
-            debug_assert!(false, "visit_icu should return Node::Icu");
-            // Return the original placeholder unchanged as fallback
+            // Return the original placeholder unchanged as a safe fallback.
             Node::IcuPlaceholder(ph.clone())
         }
     }

crates/oxc_angular_compiler/src/output/ast.rs

@@ -1389,12 +1389,7 @@ impl<'a> OutputExpression<'a> {
             OutputExpression::WrappedIrNode(_) => {
                 // WrappedIrNode expressions wrap IR expressions for deferred processing.
                 // They should be resolved during the reify phase before any cloning occurs.
-                // If we hit this, it's a compiler bug.
-                debug_assert!(
-                    false,
-                    "Cannot clone a WrappedIrExpr. WrappedIrExpr should be resolved before cloning."
-                );
-                // Return a placeholder undefined literal
+                // Return a placeholder undefined literal as a safe fallback.
                 OutputExpression::Literal(Box::new_in(
                     LiteralExpr { value: LiteralValue::Undefined, source_span: None },
                     allocator,

crates/oxc_angular_compiler/src/output/emitter.rs

@@ -113,7 +113,8 @@ impl EmitterContext {
     /// Lines is always non-empty: initialized with one element in `new()`,
     /// and `println()` only pushes (never pops below 1).
     fn current_line(&self) -> &EmittedLine {
-        debug_assert!(!self.lines.is_empty(), "lines should never be empty");
+        // Invariant: lines is always non-empty (initialized with one element in new(),
+        // and println() only pushes, never pops below 1).
         &self.lines[self.lines.len() - 1]
     }
 
@@ -123,7 +124,8 @@ impl EmitterContext {
     /// Lines is always non-empty: initialized with one element in `new()`,
     /// and `println()` only pushes (never pops below 1).
     fn current_line_mut(&mut self) -> &mut EmittedLine {
-        debug_assert!(!self.lines.is_empty(), "lines should never be empty");
+        // Invariant: lines is always non-empty (initialized with one element in new(),
+        // and println() only pushes, never pops below 1).
         let len = self.lines.len();
         &mut self.lines[len - 1]
     }