fix(angular): fix 3 compiler divergences from Angular reference

[Brooooooklyn]

1. @for duplicate let validation: Remove extra name != value condition
   that skipped implicit context variables, causing let $index = $index
   to be silently accepted instead of rejected as a duplicate.

2. $index alias handling: Change RepeaterVarNames.index from Option<Atom>
   to Vec<Atom> so multiple aliases for $index (e.g. let i = $index, j = $index) are all stored and rewritten in track expressions, matching
   Angular's Set<string> behavior.

3. @switch invalid @case unknown_blocks: Push invalid @case blocks (wrong
   parameter count) into unknown_blocks before continuing, matching
   Angular's behavior for language service autocompletion support.

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

---

Note

Medium Risk
Touches control-flow transform and repeater track-expression rewriting, which can change generated output and error surfaces for @for/@switch templates. Changes are narrow and covered by new regression tests.

Overview
Aligns @for parsing/ingestion with Angular by treating any repeated let name as a duplicate (including implicit context vars like $index), so invalid declarations now error instead of being accepted.

Fixes repeater $index alias tracking by storing all aliases (not just one) and using them when rewriting track expressions, ensuring multiple $index aliases are correctly transformed.

Updates @switch transformation so invalid @case blocks (wrong parameter count) are still recorded in unknown_blocks before being skipped, matching Angular’s language-service behavior. Adds focused regression tests for these cases.

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[Brooooooklyn]

• fix(angular): fix 3 compiler divergences from Angular reference #27 👈 (View in Graphite)
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