Add sequential inter-record Rhai helpers: prev/lag/delta/ewma

[dloss]

Motivation

• Provide ergonomic adjacent-record helpers for common sequential workflows such as latency jumps, multi-record deltas and smoothing with EWMA.
• Keep behavior deterministic and memory-bounded for streaming processing by defining clear lifecycle semantics and sequential-only mode.
• Surface resilient and strict variants to allow both permissive and fail-fast scripting styles.

Description

• Implemented a new Rhai module src/rhai_functions/inter_record.rs that provides prev, lag, delta, ewma and strict variants with validation (n >= 1, n <= 10_000, alpha in (0,1]), native-numeric delta semantics, and explicit --parallel mode errors using thread-local state.
• Integrated lifecycle hooks into the pipeline so the current event is set before per-event script stages and committed after scripts finish (including filtered events), with clear_current_event on script errors (src/pipeline/mod.rs).
• Added run-time reset calls for inter-record state at pipeline startup in both sequential and parallel contexts and worker initialization (src/runner.rs, src/parallel/worker.rs).
• Registered the new functions in the Rhai registry (src/rhai_functions/mod.rs) and added help/docs entries and reference text describing semantics and lifecycle (src/rhai_functions/docs.rs, docs/reference/functions.md, docs/reference/script-variables.md).
• Added integration tests under tests/inter_record_integration_test.rs and unit tests inside the module to cover history progression, filtered-event advancement, EWMA, contract validation, and explicit --parallel error behavior.

Testing

• Ran cargo fmt --all successfully to format code.
• Ran cargo clippy --all-targets --all-features -- -D warnings successfully with no warnings.
• Ran full test suite with cargo test, which completed successfully (all tests passed).
• Ran targeted tests cargo test rhai_functions::inter_record::tests::prev_and_lag_follow_history -- --nocapture and cargo test --test inter_record_integration_test, both of which passed.

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Codex Task