Add Section 4.5: straightness is the default, branching needs explanation

Reframes the conventional question. Discharges naturally follow the E
field gradient; branching arises from specific randomizing mechanisms
(low frequency cooling reset, simultaneous multi-channel inception,
inter-pulse memory erasure, stochastic perturbations). QCW eliminates
the randomizers, revealing the default straight-line behavior.

Insight from external reviewer discussion.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

context/branching-physics.md

@@ -251,6 +251,24 @@ The user's observation [T2] (documented in [[qcw-operation]] Section 2.3) that B
 - Competition still operates but with more noise, so side branches persist longer [T3]
 - Result: intermediate morphology on the continuum between burst (fully branched) and analog QCW (unbranched) [T3]
 
+### 4.5 Conceptual Reframing: Straightness Is the Default
+
+The conventional framing treats QCW sword sparks as the phenomenon requiring explanation: "why so straight?" The physics developed above inverts this. A discharge propagating in an electric field has a natural tendency to follow the field gradient — the field is strongest along the axis of the existing conductor (Section 3.5, Mechanism 1), and the path of lowest E_propagation threshold is straight ahead (Section 3.5, Mechanisms 2-3). Straightness is the default behavior of a discharge in the absence of perturbation.
+
+**Branching is the phenomenon that requires explanation.** It arises from specific randomizing mechanisms that disrupt the natural field-following tendency [T3]:
+
+1. **Low RF frequency** (50-100 kHz): The RF half-period (5-10 us) is long enough that thin streamers cool between individual cycles. The thermal ratchet that picks a winner keeps getting reset. The preferred conductive path diffuses and shifts between cycles.
+
+2. **High voltage, simultaneous multi-channel inception**: Burst mode at 200-600 kV launches a large crown of streamers simultaneously, all with similar initial energy. The competition timescale (~120-200 us) is comparable to the entire burst ON time (70-150 us) — the pulse ends before a winner can emerge.
+
+3. **Inter-pulse cooling erases thermal memory**: Between bursts (5-10 ms gap at 100-200 Hz), all channels cool and deionize. Every pulse starts from scratch with no inherited directional advantage. The accumulated thermal pre-conditioning that biases forward growth is destroyed.
+
+4. **Stochastic perturbations**: Electron density fluctuations, photoionization seed randomness, and turbulent mixing all create noise that pushes the discharge off-axis. These perturbations exist in all modes, but in QCW the thermal competition suppresses them faster than they grow [T3].
+
+QCW eliminates randomizers 1-3: high frequency (300-600 kHz) provides effectively continuous heating with no inter-cycle reset; low starting voltage launches few initial streamers; and sustained drive never allows the channel to cool. With the noise sources removed, the discharge does what the field geometry dictates — it grows straight outward along the E field gradient.
+
+This reframing has a practical implication [T3]: any modification that increases noise (coarser modulation, lower frequency, interrupted drive) should produce more branching, while any modification that reduces noise (smoother envelope, higher frequency, more continuous drive) should produce straighter sparks. The observed continuum from burst (fully branched) through pulse-skip (intermediate) to analog QCW (straight swords) is exactly this progression.
+
 ## 5. Capacitive Loading of Branches
 
 Each branch segment has its own shunt capacitance C_sh to ground. The total C_sh of a branched tree exceeds that of a single channel of the same main-channel length.