v0.7.9.1 — the engine was right twice; the screen was not

Gitea#22: the Division map drew every westbound train in the wrong half of
its Mainline card. `regionOfTransit` counts from the end a train entered,
which is what the collision rules ask; the map wanted "which printed box,
left to right" and used the same number, so an eastbound train came out
right by luck and a westbound one came out mirrored. It cost a collision —
Train 3 was cleared to follow T5 and ran into TX17, which the picture had
drawn ahead of T5 rather than behind it. One mirror in `view.ts`, at the
boundary the map is drawn from; the collision rules are untouched.

Gitea#21: a second tank car would not come off at a refinery, and "Blocked
— why nothing is moving" answered by describing the refinery's green box.
The real answer was Train 3's printed rule — the Express works one freight
car per location — so the refusal was correct and the panel sent the player
to spend a Freight Agent action that could not have helped. No rule
changed. The panel now names the budget, asking the reducer's own
predicate so its words cannot drift from the rule.

Both were replayed from the saves attached to the issues and verified in
the exact position each report names. The map fix is proved by mutation:
reverting the mirror fails two tests, and making the renderer ignore the
region fails a third. Every existing region test ran eastbound, where the
mirror is the identity, which is why the bug survived them.

891 tests pass, up from 884.

Closes #21
Closes #22

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E3Qk7uresKCHksdZajXCLg

test/mainline-cards.test.ts

@@ -1014,7 +1014,11 @@ describe("a train is made up to its card's consist (§8.2)", () => {
 
 describe('regions on a Mainline card (§2.1, §8.2)', () => {
   /** Put one train mid-crossing and ask the view where the map should draw it. */
-  const regionFor = (stagesTotal: number, stagesRemaining: number): { region: number; regions: number } => {
+  const regionFor = (
+    stagesTotal: number,
+    stagesRemaining: number,
+    direction: 'east' | 'west' = 'east',
+  ): { region: number; regions: number } => {
     const s = createGame({
       id: 'reg',
       seed: 4,
@@ -1027,7 +1031,7 @@ describe('regions on a Mainline card (§2.1, §8.2)', () => {
     const node = s.division.nodes.find((n) => n.kind === 'mainline');
     assert.ok(node && node.kind === 'mainline');
     const tray = [...s.trays.keys()][0]!;
-    node.transits.push({ tray, stagesRemaining, stagesTotal, direction: 'east' });
+    node.transits.push({ tray, stagesRemaining, stagesTotal, direction });
     const ml = snapshot(s, [], null).division.find((n) => n.kind === 'ml');
     assert.ok(ml, 'no Mainline node in the view');
     const t = ml!.trains.flat()[0]!;
@@ -1070,6 +1074,74 @@ describe('regions on a Mainline card (§2.1, §8.2)', () => {
       }
     }
   });
+
+  it('never leaves the card it is on, whichever way it runs', () => {
+    for (const direction of ['east', 'west'] as const) {
+      for (let total = 1; total <= 4; total++) {
+        for (let left = total; left >= 1; left--) {
+          const r = regionFor(total, left, direction).region;
+          assert.ok(r >= 0 && r <= 1, `${direction}, total ${total}, ${left} left put the train in region ${r}`);
+        }
+      }
+    }
+  });
+
+  /**
+   * GITEA#22 — A WESTBOUND TRAIN WAS DRAWN IN THE WRONG HALF OF THE CARD.
+   *
+   * `regionOfTransit` answers "how far along its crossing is this train", counted from the end it
+   * ENTERED: a train with everything still to run is in region 0. That is the right question for the
+   * collision rules, which is what the engine asks it, and both directions share the one index space.
+   *
+   * The map asks a different question — WHICH PRINTED BOX, left to right — and used the same number
+   * for it. East is right on this map and always has been, so for an eastbound train the two agree by
+   * luck: it enters at the west end, so "just entered" and "leftmost box" are the same box. A
+   * westbound train enters at the EAST end, so its region 0 is the card's RIGHT-hand box, and drawing
+   * it at index 0 put it at the left — the whole card mirrored.
+   *
+   * Reported from seed 550943578, undo 187, and it cost a collision. Three westbound trains: TX17 had
+   * just entered (2 Stages still to run, so travel index 0) and T5 was nearly across (1 Stage left,
+   * index 1). Physically TX17 was BEHIND T5 — further east, the direction they had both come from.
+   * The map drew TX17 at the left and so put it further WEST, which reads as further ahead. Asked
+   * whether Train 3 could follow Train 5 onto the card, the Superintendent said yes, and Train 3
+   * entered behind — into TX17, exactly where the rules had it and nowhere near where the map did.
+   *
+   * The engine was right throughout. Only the picture lied, so the fix is one mirror in the view and
+   * the collision rules are untouched. This is the same class of bug as the consist row at the
+   * Whistle Post (`board-svg.ts`, seed 270861860), which came out mirrored for the same reason.
+   */
+  describe('Gitea#22 — the map draws a westbound train where it actually is', () => {
+    it('mirrors a westbound train, because it entered from the east end', () => {
+      // Two-region card. Just entered, 2 Stages still to run: an eastbound train is in the WEST box
+      // and a westbound one is in the EAST box, because they came in at opposite ends.
+      assert.equal(regionFor(2, 2, 'east').region, 0);
+      assert.equal(regionFor(2, 2, 'west').region, 1);
+
+      // One Stage left, nearly across: the two swap.
+      assert.equal(regionFor(2, 1, 'east').region, 1);
+      assert.equal(regionFor(2, 1, 'west').region, 0);
+    });
+
+    it('puts the follower behind the leader, not in front of it — the seed 550943578 collision', () => {
+      // TX17 had just entered; T5 was a Stage from the far end. Both westbound, so BEHIND means to
+      // the east, which is to the right, which is the higher index.
+      const tx17 = regionFor(2, 2, 'west').region;
+      const t5 = regionFor(2, 1, 'west').region;
+      assert.ok(
+        tx17 > t5,
+        `a westbound train that has just entered must be drawn east of one that is nearly across, ` +
+          `but TX17 was drawn at ${tx17} and T5 at ${t5}`,
+      );
+    });
+
+    it('leaves an eastbound train where it has always been drawn', () => {
+      // The mirror must not disturb the direction that was right, which is every existing region test
+      // above — those all run east — and the case the printed rule was written for.
+      assert.equal(regionFor(2, 2, 'east').region, 0);
+      assert.equal(regionFor(2, 1, 'east').region, 1);
+      assert.equal(regionFor(1, 1, 'east').region, 1);
+    });
+  });
 });
 
 describe('Q13 — a train that catches the one ahead runs into it', () => {