Pollution Absorber: Improved 1.0.3

Fix multiplayer joins rejected with an event-handler mismatch.

Absorber and shared chunk references now persist in save data. Loading restores local references without registering a temporary tick handler. Existing saves migrate automatically when upgrading.

Validated on Factorio 2.0.77:

• Reproduced the 1.0.2 failure with an actual local server/client join.
• Successfully joined after upgrading, then saved/reloaded and joined again.
• Two overlapping absorbers removed 3,200 pollution over two updates after reload, within floating-point rounding.

Both host and clients must update to 1.0.3. Absorption rules and graphics are unchanged.