Snapshot WIP: solver HP epic progress, BPHX/HX physics, BMAD skill refresh.

Capture uncommitted solver robustness work (regularization, domain errors, linear solver lifecycle, tube DP/MSH), web workbench updates, and synced BMAD skills across IDE agent folders before starting BPHX pressure-drop.

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

docs/benchmarks/phase-0-baseline.md

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+# Phase-0 Performance Baseline
+
+This document records the Phase-0 benchmark results for `entropyk-solver`. All
+later Epic-1 speedup claims (NFR1 ≥2× speedup, NFR2 100% parity) are measured
+against these numbers.
+
+## Environment
+
+| Property | Value |
+|---|---|
+| Date | 2026-07-18 |
+| Host OS | Windows (x86_64) |
+| Rust toolchain | 1.89.0 (stable, pinned via `rust-toolchain.toml`) |
+| Cargo profile | bench (release) |
+| Linear algebra path | nalgebra 0.33 dense LU |
+
+## How to Reproduce
+
+```bash
+# Ensure the pinned toolchain is active
+cargo --version  # should report 1.89.0
+
+# Run the full benchmark suite
+cargo bench -p entropyk-solver
+```
+
+## Benchmark Results
+
+Results were produced by `cargo bench -p entropyk-solver` on the commit captured
+in the story frontmatter (`baseline_commit`).
+
+### LU Solve (Dense)
+
+| Benchmark | Mean time |
+|---|---|
+| `lu_solve_mock_9x9` | **425.54 ns** |
+
+Measures one Ruiz-equilibrated dense LU solve on the 9×9 Jacobian assembled
+from the deterministic mock refrigeration cycle.
+
+### Residual / Jacobian Assembly
+
+| Benchmark | Mean time |
+|---|---|
+| `residual_assembly_mock` | **420.95 ns** |
+| `jacobian_assembly_mock` | **157.47 ns** |
+
+Measures a single full-system pass of `compute_residuals` and
+`jacobian_entries` on the mock cycle.
+
+### Full Solve — Reference Cycles
+
+Each benchmark builds a real CoolProp-backed emergent-pressure R134a cycle
+(4 components, 9 unknowns) and solves it end-to-end with the fallback Newton
+solver.
+
+| Benchmark | Mean time |
+|---|---|
+| `full_solve_reference_cycle_a` | **48.64 ms** |
+| `full_solve_reference_cycle_b` | **67.06 ms** |
+| `full_solve_reference_cycle_c` | **66.78 ms** |
+
+### Sequential Batch Solve Scaling
+
+| N | Mean time |
+|---|---|
+| 1 | **56.03 ms** |
+| 2 | **118.01 ms** |
+| 4 | **240.03 ms** |
+
+This is the Phase-0 *sequential* baseline. Parallel batch solving is out of
+scope and belongs to Epic 4.
+
+## Golden Snapshots
+
+The golden regression test solves the three reference cycles and compares the
+converged state against committed canonical JSON snapshots. Run with
+`ENTROPYK_BLESS=1` to regenerate snapshots after an intentional change.
+
+| Cycle | SHA-256 (canonical JSON) |
+|---|---|
+| `golden_cycle_a.json` | `073408dede4ad4622272e9c9375d9f4823e7984ec9e54910a9790229d52af899` |
+| `golden_cycle_b.json` | `c01e7cb892ed5829926c38ad5ee1423fdfaa7d262c473a8eca46ebc4632b9142` |
+| `golden_cycle_c.json` | `6e1a77958cce1d1cf5e730505912a887c8bf4fc7dcaed820c7da482aa0ad9f98` |
+
+## Integration Corpus
+
+The NFR2 parity corpus consists of **31** pre-existing integration test files in
+`crates/solver/tests/` (including `_scratch_debug.rs` and `_tmp_analytic.rs`).
+This story added one new file (`golden_snapshot.rs`), bringing the solver test
+count to **32**. The 31 pre-existing files must continue to pass unmodified as
+later Epic-1 stories land; `golden_snapshot.rs` is part of the Phase-0 safety
+net and must also remain green.
+
+## Reproduction Commands
+
+```bash
+# Ensure the pinned toolchain is active
+cargo --version  # should report 1.89.0
+
+# Full workspace test suite
+cargo test --workspace
+
+# Solver integration corpus only (includes golden snapshot)
+cargo test -p entropyk-solver --tests
+
+# Golden snapshot regression test specifically
+cargo test -p entropyk-solver --test golden_snapshot --features coolprop
+
+# Regenerate snapshots after an intentional physics/solver change
+ENTROPYK_BLESS=1 cargo test -p entropyk-solver --test golden_snapshot --features coolprop
+
+# Benchmark suite
+cargo bench -p entropyk-solver
+
+# Lint/format gates
+cargo fmt --check
+cargo clippy --workspace --all-targets -- -D warnings
+```