docs: split conventions into CONVENTIONS.md — natural units, energy notation, SpectrumComponent design

- CONVENTIONS.md: natural units (m_e=hbar=c=1), W vs MeV energy representation,
  conversion helpers, SpectrumComponent interface, physics-specific testing
- DEVELOPMENT.md: removed physics-specific 'What to Test' section,
  references CONVENTIONS.md for physics testing guidelines

CONVENTIONS.md

@@ -0,0 +1,98 @@
+# Project Conventions
+
+This file defines the physical, mathematical, and architectural conventions that govern code correctness and consistency across the project.
+
+## 1. Natural Units System
+
+All internal representations, calculations, and definitions use natural units:
+
+```
+m_e = ℏ = c = 1
+```
+
+where `m_e` is the electron rest mass. This is the fundamental unit for energy, mass, momentum, and time throughout the codebase.
+
+## 2. Energy Representation
+
+### Core Principle
+
+Internal calculations use natural units (m_e). External interfaces (API signatures, plots, human-readable output) use eV-based units (MeV or keV).
+
+### API Signatures (Input)
+
+Use human-readable MeV for parameters that come from databases or literature:
+
+- `endpoint_MeV` — Q-value / endpoint energy in MeV
+- `detector_sigma_a_keV` — resolution in keV
+- `detector_tau_keV` — tail decay constant in keV
+
+### Internal Representation
+
+All internal computations use **total energy W in m_e units**:
+
+```
+W = T_to_W(T_MeV)   # T (kinetic) in MeV → W (total) in m_e
+W = T + 1.0          # W = T/m_e + 1  (since m_e = 1 in m_e units)
+```
+
+- `W = 1.0` corresponds to zero kinetic energy (rest mass)
+- `W > 1.0` corresponds to kinetic energy
+- All internal functions accept and operate on `W`
+- Endpoint W0 is always `T_to_W(endpoint_MeV)`
+
+### Output and Display
+
+Convert back to eV units for anything intended for human perception:
+
+- Plots: x-axis in keV or MeV (kinetic energy)
+- ASCII output, CSV export: kinetic energy in keV or MeV
+- Log messages, error messages: use keV/MeV with context
+
+```python
+kinetic_keV = (W - 1.0) * ME_KEV    # W in m_e → kinetic energy in keV
+kinetic_MeV = (W - 1.0) * ME_ME     # W in m_e → kinetic energy in MeV
+```
+
+### Conversion Helpers
+
+All conversions and constants must go through the central utilities:
+
+```python
+from beta_spectrum.utils import T_to_W, W_to_T
+from beta_spectrum.constants import ME_ME, ME_KEV, ALPHA, M_P
+```
+
+- Never hardcode physical constants — import from `constants.py`
+- Never use manual arithmetic for unit conversion — use `T_to_W()`, `W_to_T()`
+- Never mix MeV and m_e units in the same calculation
+
+## 3. SpectrumComponent Design
+
+All spectral components (Fermi function, screening, radiative correction, etc.) follow a common interface:
+
+- **Stateless or minimally stateful** — no mutable state that depends on input
+- **Vectorized** — accept and return `np.ndarray`, support array broadcasting
+- **Consistent energy variable** — always use `W` (total energy in m_e units) internally
+- **Callable interface** — implement `__call__(W)` for uniform usage
+- **Return correction factor** — output is a multiplicative factor, typically ~0.1 to 10
+
+```python
+class MyComponent(SpectrumComponent):
+    def __init__(self, param1: float, param2: float):
+        self.param1 = param1
+        self.param2 = param2
+
+    def __call__(self, W: np.ndarray) -> np.ndarray:
+        # All internal computation uses W in m_e units
+        return ...
+```
+
+## 4. Physics-Specific Testing Guidelines
+
+When writing tests for physics components, verify the following:
+
+- **Physical constraints** — values at thresholds, endpoints, and boundaries must match known physics behavior
+- **Monotonicity / positivity** — spectrum components should not produce unphysical negative values; monotonicity should match physical expectations
+- **Shape correctness** — output shape must match input shape (vectorized operation)
+- **Type safety** — components accept and return `np.ndarray`
+- **Numerical stability** — no NaN or inf in the physical energy range, except where physically expected (e.g., below kinematic threshold, near soft-photon endpoint divergence)

DEVELOPMENT.md

@@ -57,11 +57,9 @@ Branch naming: `dev/<short-description>` (e.g., `dev/recoil-correction`)
 
 ### What to Test
 
-- **Physical constraints**: values at thresholds, endpoints, and boundaries must match known physics
-- **Monotonicity / positivity**: spectrum components should not produce unphysical negative values
-- **Shape correctness**: output shape must match input shape
-- **Type safety**: components accept and return `np.ndarray`
-- **Numerical stability**: no NaN or inf in the physical energy range (except where physically expected)
+See [`CONVENTIONS.md`](CONVENTIONS.md) §4 for physics-specific testing guidelines (physical constraints, positivity, numerical stability).
+
+For general test structure, use the conventions below.
 
 ## 4. Quality Gates