fix(validate): cap GE floor tax at 5,000,000 GP per item (owner decision A)

Owner decision 2026-08-28: single global GE tax formula for all entries:
min(floor(0.02 * sell_price), 5_000_000). Apply the per-item cap inside
ge_floor_tax_error(); the margin-tag scope (flipping/merchanting) and the
generic-ge non-margin exemption are unchanged.

Regression fixtures now cover:
- fractional-tax flipping entry must FAIL (kept)
- non-margin entry with only ge + processing must PASS (kept)
- correctly-floored flipping entry must PASS (kept)
- high-value flip above the 5,000,000 GP cap (sell 300,000,000 / buy
  294,000,000 / net 1,000,000) must PASS — the old uncapped rule computed
  6,000,000 tax, demanded net 0, and rejected this valid entry (new)

Closes the governor's attempt-3 blocker on PR !7.

tools/validate.py

@@ -13,15 +13,18 @@ Runs:
        => WARNING (citation TODO — file a follow-up issue)
      - GE-margin entries (tags include flipping and/or merchanting —
        the unambiguous margin signals): the 2% GE convenience fee is
-       rounded DOWN to whole coins per item, so net_per_cycle MUST
-       equal income - floor(0.02*income) - cost.  Fractional-tax
-       arithmetic (0.98*sell - buy) is an error.
+       rounded DOWN to whole coins per item and capped at 5,000,000 GP
+       per item, so net_per_cycle MUST equal
+       income - min(floor(0.02*income), 5_000_000) - cost.
+       Fractional-tax arithmetic (0.98*sell - buy) and uncapped tax
+       (6M on a 300M sale) are errors.
        NOTE: the generic `ge` tag alone does NOT gate this rule — a
        training/processing/procurement entry may legitimately carry
        `ge` while its income/cost model has nothing to do with flip tax.
      - Regression fixtures (--selftest): a fractional-tax flipping
-       entry must FAIL; a non-margin entry carrying only `ge` plus a
-       non-flip tag must PASS.
+       entry must FAIL; an above-cap high-value flip with the
+       correctly capped-tax net must PASS; a non-margin entry carrying
+       only `ge` plus a non-flip tag must PASS.
   3. Global id uniqueness across the whole codex tree.
 
 Exit codes: 0 = clean (warnings ok), 1 = validation errors.
@@ -69,7 +72,11 @@ def ge_floor_tax_error(rel, data):
 
     Wiki: "The 2% tax rounds down to the nearest whole number, so items
     sold for below 50 coins have a tax of less than one coin" — per-item
-    net MUST use the floored tax, never 0.98*sell - buy.
+    net MUST use the floored tax, never 0.98*sell - buy. The fee is also
+    capped at 5,000,000 GP per item (per-item cap, documented in the
+    flip-candidates best_practice/risk), so tax = min(floor(0.02*sell),
+    5_000_000). Owner decision 2026-08-28: one global formula for all
+    entries (option A), no per-entry tax-model discriminator.
     """
     tags = {str(t).lower() for t in data.get("tags", [])}
     if not tags & MARGIN_TAGS:
@@ -78,13 +85,15 @@ def ge_floor_tax_error(rel, data):
     if not all(k in eco for k in ("income_per_cycle", "cost_per_cycle", "net_per_cycle")):
         return None
     sell, buy = eco["income_per_cycle"], eco["cost_per_cycle"]
-    tax = math.floor(sell * 0.02)
+    tax = min(math.floor(sell * 0.02), 5_000_000)
     expected = sell - tax - buy
     if eco["net_per_cycle"] != expected:
         return (
-            f"{rel}: net_per_cycle {eco['net_per_cycle']} != floored-tax model "
-            f"{expected} (sell {sell} - floor({sell}*0.02)={tax} - buy {buy}); "
-            f"GE tax rounds down per item — fractional 0.98*sell - buy is invalid")
+            f"{rel}: net_per_cycle {eco['net_per_cycle']} != capped floored-tax "
+            f"model {expected} (sell {sell} - min(floor({sell}*0.02), 5_000_000)"
+            f"={tax} - buy {buy}); "
+            f"GE tax rounds down per item and is capped at 5,000,000 GP per item "
+            f"— fractional 0.98*sell - buy and uncapped tax are invalid")
     return None
 
 
@@ -101,6 +110,13 @@ def run_selftest(verbose=False):
          {"income_per_cycle": 5000, "cost_per_cycle": 3000, "net_per_cycle": 2000}, False),
         ("correctly-floored flipping entry", ["flipping", "merchanting"],
          {"income_per_cycle": 244, "cost_per_cycle": 223, "net_per_cycle": 17}, False),
+        # sell 300,000,000 -> uncapped floor tax is 6,000,000; the per-item
+        # cap makes it 5,000,000, so the correct net is 1,000,000. The old
+        # uncapped rule demanded net=0 and rejected this valid entry.
+        ("high-value flip above the 5,000,000 GP per-item tax cap",
+         ["flipping"],
+         {"income_per_cycle": 300_000_000, "cost_per_cycle": 294_000_000,
+          "net_per_cycle": 1_000_000}, False),
     ]
     errors = []
     for name, tags, eco, expect_error in fixtures: