Forward test: record forecasts before their outcomes exist.

`snapshot` writes each tracked model's forecast quantiles at the seven
backtest horizons from the latest price to data/forecasts/<origin>.csv. It
refuses stale data (older than two days) and duplicate dates, so snapshots
can't be reconstructed after the fact; committing them dates them.
`forward` scores every recorded forecast whose target date has passed,
reusing the backtest's scoring (now factored out as evaluate.score).

Tracked: random_walk, drift_rw, cycle, powerlaw, plus powerlaw_ou,
powerlaw_ou_param and cycle_on_powerlaw, which development data couldn't
settle. `just weekly` runs update, snapshot and forward.

First snapshot: 2026-09-23 (BTC $84.4K). The first outcomes are due
2026-10-23.

README.md

@@ -123,6 +123,23 @@ at 2-3 years (mean PIT 0.53-0.56) but with intervals too wide (its 80% interval
 held every 2- and 3-year outcome). The holdout is now spent for these models;
 a new model needs new data to be tested honestly.
 
+### Forward test (from 2026-09-23)
+
+New data is the only honest test left, so forecasts are now recorded before
+their outcomes exist. `just snapshot` writes every tracked model's forecast
+from the latest price to `data/forecasts/<date>.csv`. It refuses if the price
+data is more than two days old or the date already has a snapshot, and the
+files are committed so version control dates them. `just forward` scores
+every recorded forecast whose target date has passed, the same way the
+backtest does.
+
+Tracked: the four registered models plus `powerlaw_ou`, `powerlaw_ou_param`
+and `cycle_on_powerlaw`, which development data couldn't settle. Snapshots
+store the forecasts themselves, so if a model's definition changes it gets a
+new name. The routine is `just weekly` (update, snapshot, score), then commit
+the new snapshot. The first 1-month outcomes arrive 2026-10-23; 1-year results
+need until late 2027, and 4-year ones until 2030.
+
 ## Usage
 
 With Nix:
@@ -133,6 +150,7 @@ just update     # fetch new daily prices from Coinbase
 just backtest   # score models on development data -> output/backtest/
 just forecast   # forecast from the latest price -> output/forecast/
 just ab         # run A/B experiments -> output/ab/
+just weekly     # new prices, a forward-test snapshot, and its scores so far
 just test
 just holdout    # score on held-out outcomes; sparingly
 ```
@@ -149,6 +167,7 @@ btcmodel/
   forecast.py   Forecast (quantiles of log price), CRPS, PIT
   evaluate.py   walk-forward backtest and summary
   experiments.py  A/B tests: hypothesis, control, variants, verdict rule
+  forward.py    forward test: record forecasts, score them as outcomes arrive
   models/       drift, volatility and shape components; register models in __init__.py
   plots.py      fan chart, skill and calibration charts
 ```