refactor: extract 6 prediction signals into Signal classes

The 803-line NationalFuelPredictionService had six private compute*Signal
methods, a private linearRegression helper, and a private disabledSignal
shape factory all crammed together. Each signal is now an independently
testable class.

- App\Services\Prediction\Signals\Signal — interface
- App\Services\Prediction\Signals\SignalContext — input value object
  (FuelType + optional lat/lng + hasCoordinates() helper)
- App\Services\Prediction\Signals\AbstractSignal — shared
  disabledSignal() and linearRegression() helpers
- TrendSignal, DayOfWeekSignal, BrandBehaviourSignal, StickinessSignal,
  RegionalMomentumSignal, OilSignal — one class each, extending
  AbstractSignal

NationalFuelPredictionService receives the 6 signal classes via constructor
injection and orchestrates them. The lat/lng null-guard for regional
momentum now lives inside RegionalMomentumSignal::compute() so the
coordinator no longer branches on coordinate presence.

Aggregation, weekly summary, and reasoning helpers stay in the service
for now — they are coupled to the public predict() output shape and are
candidates for a follow-up extraction once a stable API is locked in.

Service: 803 → 414 lines.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/Services/Prediction/Signals/Signal.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace App\Services\Prediction\Signals;
+
+interface Signal
+{
+    /**
+     * Evaluate the signal against the given context.
+     *
+     * Returns the canonical signal payload. Implementations may add extra
+     * keys beyond the base shape (e.g. trend adds slope + r_squared).
+     *
+     * @return array{
+     *     score: float,
+     *     confidence: float,
+     *     direction: string,
+     *     detail: string,
+     *     data_points: int,
+     *     enabled: bool,
+     *     ...
+     * }
+     */
+    public function compute(SignalContext $context): array;
+}