Refactor ordinary [[Set]] prototype-chain walking into a shared runtime helper

[dowdiness]

Follow-up from PR #449.

Problem

PR #449 reduced duplication between Map/Set string and symbol prototype walks, but ordinary [[Set]] prototype-chain traversal remains distributed across several object-family paths. Object, Array, Map, Set, Promise, TypedArray, and Proxy handling each encode overlapping pieces of descriptor stopping, inherited setter invocation, receiver landing, typed-array stops, Promise prototype fallback, and Proxy delegation.

That makes future correctness fixes in this area easy to apply to one path but miss in another.

Suggested work

• Inventory the existing ordinary [[Set]] walk implementations and the special cases each one owns.
• Design a shared runtime helper for descriptor/prototype traversal that can preserve existing family-specific semantics.
• Migrate callers incrementally with regression coverage for Object, Array, Map, Set, Promise, TypedArray, and Proxy cases.
• Keep the helper private/internal unless a stable public runtime API is clearly needed.

Acceptance criteria

• Shared helper covers inherited setter invocation, getter-only and non-writable blockers, writable-data stop behavior, receiver landing, Promise fallback, and Proxy delegation.
• TypedArray integer-index stop behavior remains covered.
• Existing Map/Set PR [codex] Fix Map and Set expando assignment #449 regression tests continue to pass.
• New tests demonstrate at least one shared path for string and symbol keys.
• No public API surface changes unless explicitly justified.