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perf: only grow HashMap/Map when actually inserting new entries

Optimize set_with_hash to check the grow threshold only when we need
to insert a new entry, not when updating an existing key. This avoids
unnecessary O(n) rehash operations in update-heavy workloads.

Before: grow check happens before key lookup
After: grow check happens only when inserting (key not found)

This also makes merge_in_place(self) a complete no-op without needing
the physical_equal check, though we keep that check as a fast path.

Added tests verifying that updating existing keys doesn't grow capacity.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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