Split progressive search session types

[yvette-carlisle]

Summary

• Move progressive-search session/internal storage types into types/session.rs
• Keep the public progressive-search DTO surface in types.rs
• Re-export moved internals only inside progressive_search

Validation

• cargo make fmt-rust-check
• cargo make check-rust
• cargo make lint-vstyle
• git diff --check
• cargo make lint-rust
• cargo make test-rust (370 passed, 92 skipped)
• cargo make check
• cargo make test-rust-integration (92 passed, 370 skipped)
• cargo make check-trace-gate
• Read-only scout and skeptic reviews