V1.4.0: silver and copper, with automatic change

- A character's purse holds gold, silver and copper as separate coins:
  1 gold = 10 silver = 100 copper. The money log records each entry's coin.
- Spending and giving are paid from the whole purse: coins of the kind
  named go first, then the nearest bigger coin is broken (change comes back
  in as few coins as possible), then smaller coins are counted out. A
  payment is refused only when the whole purse isn't worth enough.
- Exchange converts one coin into another at the fixed worth.
- Encounter rewards and gifts between characters can be in any coin; the
  party overview shows the whole purse.
- All packages at version 1.4.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/CHARACTERS.md

@@ -509,17 +509,35 @@ character already has adds to the stack. Items have no effect on any stat. An
 item can carry optional dice (a potion's healing, a bomb's damage) that grow
 with its level exactly like a weapon's (section 10).
 
-**Money** is a single gold balance with a log behind it.
+**Money** is a purse of three coins, each counted on its own, with a log
+behind it.
 
-- The sheet's Money card shows the balance and every change to it: the amount,
-  what it was for, who recorded it, when, and the balance afterwards.
-- The **owning player and the DM** can both record a gain or a spend. Other
-  members can see the log but not add to it.
-- The balance **cannot go below zero**; a spend larger than the balance is
-  refused.
-- The log is the only way the balance changes. Entries cannot be edited or
+| Coin | Worth |
+|---|---|
+| Gold (gp) | 1 gold |
+| Silver (sp) | 0.1 gold — 10 to the gold |
+| Copper (cp) | 0.01 gold — 100 to the gold, 10 to the silver |
+
+- The sheet's Money card shows the three balances, what the purse is worth in
+  gold altogether, and every change: the amount and coin, what it was for, who
+  recorded it, when, and that coin's balance afterwards.
+- The **owning player and the DM** can both record a gain or a spend, choosing
+  the coin. Other players cannot see the purse or the log.
+- **Spending makes change for you.** A spend (or a gift to another character)
+  is paid from the whole purse. Coins of the kind named go first; if there
+  aren't enough, the nearest bigger coin is broken and the change comes back
+  in as few coins as possible; if there are no bigger coins, smaller ones are
+  counted out. Spending 3 silver from a purse of 5 gold leaves 4 gold and 7
+  silver. Each coin that moved gets its own log entry, marked "with change".
+- A spend is refused only when the **whole purse** isn't worth enough.
+- **Exchange** turns coins of one kind into another at the worth above, and
+  logs both sides. It moves exactly the coins named and has to come to whole
+  coins: silver into gold goes in tens, copper into silver in tens, copper
+  into gold in hundreds.
+- The log is the only way a balance changes. Entries cannot be edited or
   deleted; to correct a mistake, record the opposite entry.
-- There is one currency (gold). Silver, copper and so on are not tracked.
+- Starting gold (a campaign setting) is gold only. Encounter rewards and gifts
+  between characters can be in any coin.
 
 ### Giving to another character