fix: propagate ?. optional chaining to method call nodes in native parser

[cs01]

Summary

• The native parser (parser-native/transformer.ts) never propagated ?. from member expressions to MethodCallNode.optional, causing the native compiler to skip null-check branching for all obj?.method() calls
• This was the root cause of the "if-drop" bug — the dropped optcall null-check blocks caused IR divergence between JS and native compilers (956 out of 2036 functions differed), leading to self-hosting failures
• Added loc and optional fields to all MethodCallNode creation sites in the native parser for consistent struct layout

Test plan

• New test fixture classes/optional-method-call.ts verifies ?. returns null when object is null
• All 437 tests pass
• verify:quick passes (Stage 0 + Stage 1 self-hosting)

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