Implement handle store with TTL and expand

[dgenio]

Summary

Implement in-memory handle store with TTL eviction and structured expand queries.

HandleStore API

• store(data, metadata, ttl_seconds=600) → Handle — unique handle_id, stores data + expiry
• retrieve(handle_id) → (data, metadata) — raises HandleNotFound or HandleExpired
• evict_expired() — remove all expired entries
• Handle.data_shape: {"type": "list"|"dict"|"str", "count": N, "fields": [...], "size_bytes": N}

Expand queries — expand(handle_id, query) → Frame

Apply in order: filter → field selection → pagination → budget enforcement

• {"offset": N, "limit": M} — pagination for list data
• {"fields": ["a", "b"]} — field selection for list-of-dicts
• {"filter": {"key": "value"}} — basic equality filter for list-of-dicts
• Combinations work together: filter first, then select fields, then paginate
• Non-applicable queries (e.g., pagination on a dict) are silently ignored
• Result wrapped in a Frame with budgets applied

Acceptance criteria

• Store + retrieve round-trip works
• Expired handle raises HandleExpired
• Missing handle raises HandleNotFound
• evict_expired removes only expired entries
• Expand pagination returns correct slice
• Expand field selection returns only requested fields
• Expand filter returns matching records only
• Combined query: filter → fields → pagination