Implement Dict builtins and related fixes#9414
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Polymorphic functions whose return type is a nominal containing rigids (like `Dict(k, v)` in `Dict.from_list`) crashed the interpreter with an `e_tag: expected tag_union but got rt=err` error. Three interacting issues in runtime type translation: - `collectRigidsFromType` didn't recurse into nominal_type args, so rigids nested inside another nominal (e.g. k, v inside `List((k, v))` in Dict's backing) were never found, leaving the backing disconnected from the call site's type args. - Nested nominal translations cleared `translate_rigid_subst`, clobbering the outer scope's mappings — now snapshot/restore around each nominal_type translation. - `putFlexTypeContext` only bumped the polymorphic-context generation on value changes, not on new entries. Cached translations from before a polymorphic body's `propagateFlexMappings` ran were returning stale rigid translations; bump on new entries too. Add a snapshot test for `Dict.from_list`. Update the `List.repeat - empty case` test, which previously relied on the stale-cache behavior to produce a `.list_of_zst` layout — with the fix the element type resolves correctly, so the empty list has a proper `.list` layout with the I64 element layout. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
When resolving a method's type variables against the receiver, the interpreter walked the return type and receiver type in parallel — fine for `T(s) -> T(s)`, but for `Dict(k, v) -> List(v)` it aligned `List`'s single arg with `Dict`'s first arg, leaving `v -> Str` in flex_type_context and breaking the subsequent match on `Pairs(list)`. Gate the return-to-receiver propagation on a matching nominal head so the misalignment can't happen. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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