Optional shared pure helpers for dual projection backends

[patrickleet]

Implements [[tasks/rust-less-context-21]]

Stacked on #127 / tasks--graphql-qs-epic.

Summary

• Added src/projection_protocol/store/backend_helpers.rs for shared pure backend helpers: checked_next, table_model_name, and change_kind_for_mutation.
• Updated SQLx and in-memory projection protocol modules to use the shared helpers through the existing crate::projection_protocol path.
• Kept backend trait impls and storage-specific logic in backend modules; no intentional behavior, wire format, SQL semantic, or error-code changes.

Verification

• cargo test --lib projection_protocol (55 passed, 0 failed, 413 filtered)
• cargo test --lib sqlx_repo (0 passed, 0 failed, 468 filtered)
• cargo test --lib in_memory_repo (32 passed, 0 failed, 436 filtered)
• cargo test --features sqlite --lib (491 passed, 0 failed, 0 filtered)

Rebase

• git fetch origin tasks--graphql-qs-epic && git rebase origin/tasks--graphql-qs-epic reported the branch was up to date; verification was rerun afterwards.

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