refactor(semantic): introduce TypeMember abstraction for unified struct/tuple member access

[orizi]

Summary

Introduces TypeMember and TypeMemberKind as a unified abstraction over named struct members and positional tuple elements. Adds a new type_members query (backed by type_members_query) that returns a type's accessible members keyed by their access name — struct member names for structs, and stringified indices ("0", "1", ...) for tuples. Updates EnrichedTypeMemberAccess and EnrichedMembers to use TypeMember instead of Member, and updates member_access_expr and get_enriched_type_member_access to go through the new type_members query rather than calling concrete_struct_members directly.

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Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

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Why is this change needed?

Previously, member access resolution only handled concrete struct types by calling concrete_struct_members directly, with no shared abstraction for tuple element access. This made it impossible to unify member access logic across structs and tuples, and left tuple element access (e.g., my_tuple.0) unsupported through the enriched member access path.

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What was the behavior or documentation before?

Member access enrichment operated exclusively on concrete_struct_members, returning Member values. Tuple types returned an empty map by default, and there was no TypeMember abstraction to distinguish between struct members and tuple elements.

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What is the behavior or documentation after?

A new TypeMember struct and TypeMemberKind enum unify struct members and tuple elements. The type_members query returns members for both structs and tuples. The enriched member access path now uses TypeMember throughout. Tuple element access via numeric index names is structurally supported, with full handling gated behind a TODO referencing issue #7608.

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Related issue or discussion (if any)

#7608

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Additional context

The member_access_expr function currently returns Unsupported for TypeMemberKind::TupleElement accesses, as noted by the TODO(#7608) comment. The groundwork laid here is intended to be completed in that follow-up.

[reviewable-StarkWare]

This change is 

[orizi]

• feat(syntax,semantic,lowering): support tuple struct syntax (struct Foo(T1, T2)) #9885
• feat(semantic): support tuple member access via v.0 syntax (#7608) #9884
• refactor(semantic): introduce TypeMember abstraction for unified struct/tuple member access #9883 👈 (View in Graphite)
• main

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