⚡ Bolt: Optimize dictionary defaults in hot loops

[Wenbobobo]

💡 What: Replaced dict.setdefault(key, complex_default) with an explicit if key not in aggregates: check in src/geometry/hull_kv.py's _rebuild_if_needed method.
🎯 Why: Using dict.setdefault inside a hot loop forces the Python interpreter to eagerly evaluate the default value on every iteration, even if the key already exists. In this case, the default was a dictionary containing a list comprehension ([Fraction(0) for _ in value]), leading to significant object creation overhead per loop iteration.
📊 Impact: Reduces object creation and fraction initialization overhead during rebuilds by roughly ~50% (measured in local scratchpad, execution time dropped from 0.74s to 0.35s).
🔬 Measurement: Verified via local benchmark script simulating the specific setdefault pattern with Fraction instantiation vs the explicit branching logic. All relevant tests (tests/test_geometry_hardmax.py) pass without regressions.

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PR created automatically by Jules for task 4047790529365782008 started by @Wenbobobo

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[Copilot]

Pull request overview

Optimizes a performance-critical aggregation loop in the 2D geometry cache rebuild path by replacing dict.setdefault(..., complex_default) with lazy initialization, avoiding unnecessary object creation during rebuilds.

Changes:

• Replaced aggregates.setdefault(key, {...}) with an explicit if key not in aggregates: initialization in HullKVCache._rebuild_if_needed.
• Added a short performance note to .jules/bolt.md documenting the “avoid eager setdefault defaults in hot loops” guideline.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/geometry/hull_kv.py	Avoids eager default construction inside the rebuild hot loop by lazily initializing per-key aggregate buckets.
.jules/bolt.md	Documents the performance guideline motivating the change for future reference.

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[Copilot]

In this hot loop, if key not in aggregates: followed by bucket = aggregates[key] performs two dict lookups on the common (key-present) path. For maximum performance, consider a single-lookup pattern (e.g., try/except KeyError or bucket = aggregates.get(key) with a sentinel) so the steady-state path only hashes the key once per iteration.

Suggested change

	# Performance optimization: explicit check to avoid eagerly evaluating default dict
	if key not in aggregates:
	aggregates[key] = {
	"value_sum": [Fraction(0) for _ in value],
	"count": 0,
	"entry_indices": [],
	}
	bucket = aggregates[key]
	try:
	bucket = aggregates[key]
	except KeyError:
	bucket = {
	"value_sum": [Fraction(0) for _ in value],
	"count": 0,
	"entry_indices": [],
	}
	aggregates[key] = bucket