Migrate evaporator and flash state handling to StateLayout

[bernalde]

Parent: #118

Depends on the StateLayout infrastructure sub-issue.

Scope

Migrate isothermal and adiabatic flashes, Evaporator, and ContinuousEvaporator away from repeated positional packing and slicing.

Acceptance criteria

• Use one declared layout for initialization, model evaluation, events, and results.
• Remove duplicate hard-coded state ordering in the migrated paths.
• Add regression coverage for documented operating modes.
• Demonstrate unchanged numerical results except for separately tracked defects.