State-spec multiphase flash (PT + PH + PS)
Research-grade — validation in progress.
PT (pressure-temperature) flash is shipped through the existing PR-EOS pipeline; **v9 Gap A** adds enthalpy-specified (PH) and entropy-specified (PS) flash via `src/lib/reservoir/phPsFlash.ts` (`flashPH` / `flashPS` + Joule-Thomson + isentropic-expansion helpers). DARTS-flash-aligned Helmholtz-form negative flash with Newton on the (T, V) target. 8 drift-guards locking PT≡PH≡PS round-trip on the calibration mixture, monotone JT cooling across a CO₂ choke, and ideal-gas isentropic-expansion identity.
Wire PH/PS as an opt-in entry point on `SolverPreferencesCard` so choke / geothermal-injector / CO₂ blow-down advisors swap from PT to PH (constant-enthalpy expansion) or PS (reversible-adiabat) without rewriting call-sites. Two-phase Span-Wagner-grade hydrate-water-CO₂ stability re-call is the next honest gate (separate from VdW-Platteeuw equilibrium, which is shipped — see `state-spec-flash` sibling row).
No phase markers extracted from this row's prose yet.
- No published error band yet — use with engineering judgment.