feat(api) add timestamp function to node api

[oceanusxiv]

Summary

• Add timestamp() method to the Rust DoraNode that returns the current HLC timestamp from the node's internal clock
• Expose timestamp() on the Python Node class, returning a timezone-aware UTC datetime.datetime
• Make the cached datetime_module() helper public in dora-operator-api-python so the node crate can reuse it instead of calling PyModule::import on every invocation

Motivation

DoraNode already maintains an internal uhlc::HLC clock used to timestamp every outgoing message, but there was no way for user code to read the current clock value directly. This is useful for:

• Measuring processing latency by comparing the current clock against an input event's timestamp
• Correlating log entries or external events with the same logical clock used by dora's data plane
• Profiling node processing time without introducing a separate clock source

Changes

File	Change
apis/rust/node/src/node/mod.rs	Add pub fn timestamp(&self) -> uhlc::Timestamp
apis/python/node/src/lib.rs	Add timestamp() PyO3 method returning datetime.datetime (UTC)
apis/python/operator/src/lib.rs	Make datetime_module() pub for cross-crate reuse
apis/python/node/dora/__init__.pyi	Add type stub for timestamp()

Usage

Rust:

let node = DoraNode::init_from_env()?;
let ts = node.timestamp();
println!("Current HLC time: {ts}");

Python:

from dora import Node

node = Node()
print(f"Current HLC time: {node.timestamp()}")

for event in node:
    if event["type"] == "INPUT":
        latency = node.timestamp() - event["metadata"]["timestamp"]
        print(f"Processing latency: {latency}")

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[heyong4725]

Closing in favor of #1877, which rescues this against current main. Cherry-picks cleanly, plus expanded docstrings (the HLC-vs-SystemTime guidance from your motivation is now in the rustdoc) and a regression test pinning the node-clock-not-fresh-clock contract. Co-authored-by credit preserved. Thanks for the original work!