docs: initialize project wiki from production hardening session

12 pages covering architecture, findings, and metrics from the
rotatingMachine + machineGroupControl hardening work:

- Overview: node inventory, what works/doesn't, current scale
- Architecture: 3D pump curves, group optimization algorithm
- Findings: BEP-Gravitation proof (0.1% of optimum), NCog behavior,
  curve non-convexity, pump switching stability
- Metrics: test counts, power comparison table, performance numbers
- Knowledge graph: structured YAML with all data points and provenance
- Session log: 2026-04-07 production hardening
- Tools: query.py, search.sh, lint.sh

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

wiki/findings/ncog-behavior.md

@@ -0,0 +1,42 @@
+---
+title: NCog Behavior and Limitations
+created: 2026-04-07
+updated: 2026-04-07
+status: evolving
+tags: [rotatingMachine, NCog, BEP, efficiency]
+sources: [nodes/rotatingMachine/src/specificClass.js]
+---
+
+# NCog — Normalized Center of Gravity
+
+## What It Is
+NCog is a 0-1 value indicating where on its flow range a pump operates most efficiently. Computed per tick from the current pressure slice of the 3D pump curve.
+
+```
+BEP_flow = minFlow + (maxFlow - minFlow) * NCog
+```
+
+## How It's Computed
+1. Pressure sensors update → `getMeasuredPressure()` computes differential
+2. `fDimension` locks the 2D slice at current system pressure
+3. `calcCog()` computes Q/P (specific flow) across the curve
+4. Peak Q/P index → `NCog = (flowAtPeak - flowMin) / (flowMax - flowMin)`
+
+## When NCog is Meaningful
+NCog requires **differential pressure** (upstream + downstream). With only one pressure sensor, fDimension is the raw sensor value (too high), producing a monotonic Q/P curve and NCog = 0.
+
+| Condition | NCog for H05K | NCog for C5 |
+|-----------|--------------|-------------|
+| ΔP = 400 mbar | 0.333 | 0.355 |
+| ΔP = 1000 mbar | 0.000 | 0.000 |
+| ΔP = 1500 mbar | 0.135 | 0.000 |
+| ΔP = 2000 mbar | 0.351 | 0.000 |
+
+## Why NCog = 0 Happens
+For variable-speed centrifugal pumps, Q/P is monotonically decreasing when the affinity laws dominate (P ∝ Q³). At certain pressure levels, the spline interpolation preserves this monotonicity and the peak is always at index 0 (minimum flow).
+
+## How the machineGroupControl Uses NCog
+The BEP-Gravitation algorithm seeds each pump at its BEP flow, then redistributes using slope-based weights + marginal-cost refinement. Even when NCog = 0, the slope redistribution produces near-optimal results because it uses actual power evaluations.
+
+> [!warning] Disproven: NCog as proportional weight
+> Using NCog directly as a flow-distribution weight (`flow = NCog/totalNCog * Qd`) is wrong. It starves pumps with NCog = 0 and overloads high-NCog pumps. See `calcBestCombination` in machineGroupControl.