Scope provider provenance by source

[luca-ctx]

Summary

• scope provider capture source/event/file-touch identities by provider session, source format, and raw source path
• keep upgraded stores idempotent by reusing existing legacy provider event/file-touch identities when present
• allow archive imports to contain multiple capture sources for the same provider session
• widen source-scoped event seq layout and add regressions for large provider indices

Validation

• cargo fmt --check
• cargo test -p ctx-history-capture provider_
• cargo test -p ctx-history-capture
• cargo test -p ctx-history-store archive_import_allows_multiple_capture_sources_for_same_provider_session
• cargo test -p ctx-history-store
• cargo clippy -p ctx --all-targets -- -D warnings

Review

• addressed subagent review findings around legacy reimport duplication, event seq collisions, and archive import source uniqueness