fix HeatExchangerNetwork cache_network: keep stored order, match units by identity

[sarangbhagwat]

Problem

cache_network=True on HeatExchangerNetwork was silently broken in two independent ways in HeatExchangerNetwork._cost():

1. It never fired when auxiliary heat exchangers share IDs — the norm in biorefineries (several condenser/reboiler units). The cache-hit test compared sorted(hxs, key=ID) of the current utilities (unit order) with the stored list (duty-sorted synthesis order); ties keep input order, so the two lists differed and the network was re-synthesized every simulation.
2. When it did fire (unique IDs), it paired the wrong streams. It rebuilt hxs in unit order and paired hxs[i] with stream_life_cycles[i], which synthesize_network built for the duty-sorted list. Each life cycle was seeded with the wrong stream. The final consistency check compared against the same wrongly paired unit, so it passed: every HXprocess ended with Q = 0, the heat-utility ratio went to 1.0, and the incremental HX cost collapsed — with no warning (class doctest system: 0.82 → 1.00).

Fix

Treat the stored synthesis order as the source of truth. The cache-hit test compares identity sets of units (HeatUtility objects are recreated on every simulation, so the unit is the only stable key), and the cached branch iterates self.original_heat_exchangers in stored order, looking up each unit's live HeatUtility. All downstream index-based pairing is then correct by construction.

Also:

• the bare except: around the cache consistency check now catches AssertionError and reports the reason;
• _energy_balance_error_contributions no longer raises TypeError when ignored is None.

Validation

• New tests/test_hxn.py: the cache fires and matches fresh synthesis on the doctest system, after a 1 % feed perturbation, and with duplicate auxiliary IDs; ignored=None smoke test.
• Sugarcane biorefinery with all 10 utility streams: cached HXN runs in 0.11 s vs 0.31 s, identical results.
• HeatExchangerNetwork class doctest output unchanged.
• Full suite (pytest . --disable-numba=1 -m "not slow"): same pre-existing failure set as on unmodified master (thermosteam/biorefineries stack drift, missing optional deps, tutorial notebooks); nothing new.

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