Split ingestion profile default storage

[yvette-carlisle]

Summary

• Add ingestion_profiles::storage::defaults as a private child module.
• Move default profile pointer persistence and default profile seeding out of storage.rs.
• Preserve existing storage::... call sites through a narrow pub(super) facade.

Validation

• cargo make fmt-rust
• cargo make check-rust
• cargo make lint-vstyle
• cargo make lint-rust
• cargo make test-rust
• cargo make check
• cargo make test-rust-integration
• cargo make check-trace-gate
• git diff --cached --check

Review

• Read-only scout selected this split as a low-risk storage boundary.
• Read-only skeptic review found the Rust move behavior-preserving; its only blocking note was resolved by staging the new defaults.rs module before commit.