Add Obsidian knowledge base (docs/) and update README with docs structure

docs/02-phase-space.md

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+---
+title: "02 – Phase Space"
+date: 2026-04-26
+tags:
+  - beta-spectrum/correction/phase-space
+status: active
+aliases: [Phase Space, Baseline Spectrum]
+cssclasses: []
+related_components: [[01-fermi-function]], [[07-recoil-effects]]
+---
+
+# Phase Space — Baseline Spectral Shape $p W (W_0 - W)^2$
+
+## The Unmodified Spectrum
+
+The baseline β spectrum shape without any corrections is pure phase space:
+
+$$\frac{dN}{dW}\bigg|_{\text{PS}} = p W (W_0 - W)^2 \tag{4, first factor}$$
+
+where:
+- $p = \sqrt{W^2 - 1}$ — electron momentum ($m_e c$ units)
+- $W$ — total electron energy ($m_e c^2$ units)
+- $W_0$ — endpoint (Q-value + $m_ec^2$)
+
+## Neutrino Mass Effects
+
+If the neutrino has non-zero mass $m_\nu$, the spectrum is modified near the endpoint:
+
+$$p \to p_e = \sqrt{W^2 - 1}, \quad p_\nu = \sqrt{(W_0 - W)^2 - m_\nu^2}$$
+
+The phase space factor becomes:
+$$(W_0 - W)\sqrt{(W_0 - W)^2 - m_\nu^2}$$
+
+This produces a characteristic distortion near $W = W_0 - m_\nu$, used in neutrino mass experiments (e.g., KATRIN with tritium β decay).
+
+## Kurie Plot
+
+The phase space subtracted spectrum gives the **Kurie function**:
+
+$$K(W) = \sqrt{\frac{N(W)}{p W F(Z,W)}} \approx W_0 - W \quad (\text{linear for allowed transitions})$$
+
+Deviations from linearity near the endpoint indicate:
+- Non-zero neutrino mass
+- Weak magnetism / recoil effects ([[07-recoil-effects]])
+- New physics (Fierz term [[Hayen2017_summary#The Fierz Interference Term]])
+
+## Calculator Implementation
+
+> [!tip] Code mapping
+> Module: `beta_spectrum/components/phase_space.py` → `PhaseSpace(W0)` class
+> 
+> Simple evaluation of $p \times W \times (W_0 - W)^2$ with masking near endpoint for numerical safety. Optionally includes neutrino mass support.
+
+## Energy Range
+
+The phase space factor:
+- Vanishes at $W = 1$ ($T = 0$, the threshold) — goes as $(W-1)^{1/2}$ via momentum
+- Vanishes at $W = W_0$ (the endpoint) — quadratic behavior from neutrino energy
+- Peaks roughly at $W \approx W_0 / 3$ to $W_0 / 2$ depending on $Z$
+
+## Code API
+
+```python
+from beta_spectrum import PhaseSpace, T_to_W
+
+ps = PhaseSpace(W0=T_to_W(5.0))  # endpoint = 5 MeV
+values = ps(W_grid)               # returns p·W·(W₀-W)²
+```