Add multi-base chain modelling, fix price-band-aware demand/supply, tranche-split sell tool

- puga/network.py + tools/network.py: model a multi-base chain (e.g. mine LST at one base,
  ship it, consume it at another). Simulates each base independently, nets a transferred
  material's producer-surplus against consumer-need, charges real freight only on what's
  moved, cm_free for a founding covered by a Core Module Kit. Worked example in plans/chains/.
- puga/saturation.py: real price-band filtering. FIO's order_book NarrowPriceBandLow/High and
  WidePriceBandLow/High match APEX's own displayed Price Band exactly (verified live) - an
  order outside it is a stale artifact, not just uncompetitive. Added in_band() and
  effective_demand(); effective_supply() gained the same hard band filter alongside its
  existing soft vwap-proximity filter (renamed that param mult to free up `band`). Wired into
  tools/scan.py stage 2 in place of the raw, unfiltered demand figure. Was flagged as an
  unimplemented refinement in saturation-design.md since the original design review.
- tools/sell.py: self-contained tranche split (aggressive tranche capped at a volume quantile,
  median normally or 80th pct with --tight when a payment is imminent and stockout risk
  outweighs margin; patient tranche priced under the next competitor tier). Instant-bid
  comparison moved behind --show-bid (off by default).
- Docs and roadmap updated accordingly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_sell.py

@@ -0,0 +1,34 @@
+import importlib.util
+from pathlib import Path
+
+spec = importlib.util.spec_from_file_location("sell", Path(__file__).resolve().parent.parent / "tools" / "sell.py")
+sell = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(sell)
+
+TRADES = [5, 10, 15, 20, 25, 30, 35, 40, 45, 50]   # evenly spaced: median 25, 80th pct 40
+
+
+def test_quantile_median_and_80th():
+    assert sell.quantile(TRADES, 0.5) == 25
+    assert sell.quantile(TRADES, 0.8) == 40
+    assert sell.quantile([], 0.5) == 0.0
+
+
+def test_tranche_split_caps_aggressive_at_quantile_not_full_stock():
+    aggr, patient = sell.tranche_split(60, TRADES, tight=False)
+    assert aggr == 25 and patient == 35                # median cap
+
+
+def test_tranche_split_tight_uses_higher_quantile():
+    aggr, patient = sell.tranche_split(60, TRADES, tight=True)
+    assert aggr == 40 and patient == 20                # 80th pct cap, bigger aggressive tranche than non-tight
+
+
+def test_tranche_split_no_split_when_stock_below_quantile():
+    aggr, patient = sell.tranche_split(11, TRADES, tight=False)
+    assert aggr == 11 and patient == 0                 # whole stock fits under the median, nothing held back
+
+
+def test_tranche_split_falls_back_to_full_qty_with_no_history():
+    aggr, patient = sell.tranche_split(8, [], tight=False)
+    assert aggr == 8 and patient == 0