fix: excessive cloning in runSubscribersWithMutation

[daohoangson]

This PR fixes #883

The runSubscribersWithMutation function has a critical performance bottleneck causing long execution time with the potential to hang the browser completely.

Impact

• The example dojo app has 26 active subscribers during streaming
• Message arrays grow from 0 to 42 messages during a 742KB test run with ~200 events
• Current execution time: 810ms

Traffic	Events	Trace

Root Cause

This is in the hot path, called 2× per streaming event (once for onEvent, once for specific event handler), but it:

• Clones per subscriber even if they do not mutate
• Does expensive change detection via JSON.stringify()

The Fix

• Move cloning from inputs to outputs
• Use reference equality to identify changes

Expected Results

• New execution time: ~53ms (15x faster)
• Eliminate serialization bottleneck visible in flame graphs

[Copilot]

Pull request overview

This PR addresses a critical performance bottleneck in runSubscribersWithMutation that was causing execution times of ~810ms with potential to hang the browser. The optimization achieves a 15x performance improvement by eliminating unnecessary cloning and replacing expensive JSON.stringify comparisons with reference equality checks.

Key changes:

• Moved cloning from input parameters (per-subscriber) to output mutations (only when mutations occur)
• Replaced JSON.stringify equality checks with reference equality checks for change detection
• Maintained functional correctness while dramatically reducing clone operations in the hot path

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

This optimization removes the defensive cloning that protects against subscribers mutating the input parameters directly. While subscribers should only mutate via return values, passing the same reference to multiple subscribers means that if a subscriber mutates the messages or state parameters in-place (even accidentally), those mutations will leak to subsequent subscribers.

The old approach cloned the inputs for each subscriber, providing isolation. Consider whether this risk is acceptable, or if there should be documentation/runtime checks to enforce that subscribers don't mutate inputs directly.

Suggested change

	const mutation = await executor(subscriber, messages, state);
	const messagesForSubscriber = structuredClone_(messages);
	const stateForSubscriber = structuredClone_(state);
	const mutation = await executor(
	subscriber,
	messagesForSubscriber,
	stateForSubscriber,
	);

[daohoangson]

This is intentional.

[daohoangson]

The failing job is unrelated to the change in this PR, reported separately here #897