Complete Story 0.2 phantom-gradient regularization standard.

Add analytic derivatives and C1 tests for smooth_max/min/clamp, document epsilon/width guidance, and declare flow_regularization as the HX reference pattern without changing component physics.

Co-authored-by: Cursor <cursoragent@cursor.com>

crates/components/src/heat_exchanger/flow_regularization.rs

@@ -1,5 +1,9 @@
 //! C¹/C∞ flow regularization for zero-flow-safe heat-exchanger residuals.
 //!
+//! **Reference specialized pattern** (Story 0.2): keep these HX helpers here;
+//! the canonical general toolkit is [`entropyk_core::smoothing`]. See
+//! `docs/components/phantom-gradient-regularization.md`.
+//!
 //! Zero mass flow is a **valid** operating state (circuit staging, isolation,
 //! Newton trial steps). Hard branches `if |m| < ε { Q = 0 }` create Jacobian
 //! discontinuities and can leave energy residuals under-ranked.