Performance: Reduce redundant network_hash calls in selection

[e-lo]

Parent Issue

Part of #442 (Performance: Sub-second edits for large networks)

Problem

The selection code in network_wrangler/roadway/selection.py calls network_hash multiple times during a single operation:

# selection.py:149 - On selection creation
self._stored_net_hash = copy.deepcopy(self.net.network_hash)

# selection.py:345-346 - When getting selected links (called twice!)
if self._selected_links_df is None or self._stored_net_hash != self.net.network_hash:
    self._stored_net_hash = copy.deepcopy(self.net.network_hash)

Even with an efficient hash implementation, these redundant checks add overhead.

Proposed Solution

1. Single hash check per operation: Pass a "transaction context" that tracks whether the network has changed
2. Lazy invalidation: Only check hash when explicitly requested, not on every property access
3. Remove redundant copy.deepcopy: String hashes don't need deep copying

# Before
self._stored_net_hash = copy.deepcopy(self.net.network_hash)

# After  
self._stored_net_hash = self.net.network_hash  # Strings are immutable

Files to Modify

• network_wrangler/roadway/selection.py
• network_wrangler/transit/selection.py (if applicable)

Expected Improvement

Reduces hash-related overhead from 3 calls to 1 call per operation when combined with #443.