v0.7.4 — Red Flags hold a train out of your Limits (Gitea#19)

"If played, asked FLAG EAST or FLAG WEST. That stops all trains from entering your
limits from that direction (i.e. Flag East holds westbound trains). You can do this
if you see a problem or wish to complete switching."

REPLACES the old rule outright, per Jesse's call. Red Flags used to be played on a
stopped train out on the Mainline and protected it from a rear-ender: offered 4,212
times and played 4 across 600 games, a mechanic nobody used, and ABS Signals already
does that job better. The flag is now planted on one side of your own district and
holds the next train arriving from that side.

SPENT ON THE TRAIN IT STOPS. One card, one train, so there is no lifting action to
build, nothing to forget, and a flag cannot quietly strangle the Division. The held
train loses one Mainline Phase and comes in on the next — it buys a Stage to clear
the lead, which is what "wish to complete switching" asks for.

PLAYABLE OUT OF PHASE, which is the other half of the issue: when an arrival would
certainly collide and the district's owner holds the card, the phase breaks in and
asks. Offered ONLY to somebody holding one — a prompt with a single button is not a
choice, and it would leak that a collision is coming. The danger is read from §8.3's
own two triggers rather than restated, so the prompt cannot offer a flag against a
collision that will not happen.

Built on the decision union Gitea#5 introduced: this adds a `redFlag` case and
nothing else structural.

THE BOT STILL NEVER PLAYS IT, AND I MEASURED RATHER THAN ASSUMED. It now takes the
out-of-phase prompt unconditionally — the engine has already established the danger,
so there is nothing left to judge — and over 200 solitaire games `redFlagsSet` fires
ZERO times. The prompt needs an arrival that would collide (0.14 per game, about one
game in seven) to coincide with holding the card from a three-card hand out of 121.
So the anomaly exemption in sim.test.ts stays, but its comment no longer claims the
bot is unwilling: it is measuring deck luck. What is left to fix is the half of the
card a human would use, planting a flag on purpose to buy switching time, and TODO.md
now says that instead of the old finding.

A BUG WORTH RECORDING, because the next interruption will meet it too: the flag was
originally taken down in a `reduce` case, which never fires for an event advance.ts
emits — the phase driver mutates state and then describes it. The flag stayed up and
held every train that came. test/events.test.ts's unreduced-event registry is what
makes that class of mistake visible, and `redFlagSpent` is on it deliberately now,
with the reasoning.

858 tests pass.

Closes #19

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EAgJSmeV8zrMh55Mj85ESb

src/engine/intents.ts

@@ -120,10 +120,24 @@ export type Intent =
    */
   | { type: 'mainline.modify'; cardId: CardId; node: number }
   /**
-   * Red Flags — protect a stopped train. The flagged train cannot be hit; an approaching train is
-   * held instead of colliding.
+   * §Q, RED FLAGS (Gitea#19) — plant a flag on one side of your own district.
+   *
+   * "If played, asked FLAG EAST or FLAG WEST. That stops all trains from entering your limits from
+   * that direction (i.e. Flag East holds westbound trains). You can do this if you see a problem or
+   * wish to complete switching."
+   *
+   * `side` names the side of the district the flag goes on, so a train arriving from that side is
+   * held. It REPLACES the old rule, which was played on a stopped train out on the Mainline and
+   * protected it from a rear-ender: measured at 4,212 offers and 4 plays across 600 games, a
+   * mechanic nobody used. ABS Signals already protects a train standing on a Mainline card.
    */
-  | { type: 'maneuver.redFlags'; cardId: CardId; trayId: TrayId }
+  | { type: 'maneuver.redFlags'; cardId: CardId; side: Direction }
+  /**
+   * The same card, played OUT OF PHASE at the moment of danger (Gitea#19) — "COLLISION RISK! FLAG
+   * AGAINST T2?". Answers a pending `redFlag` decision; `flag: false` declines and lets the
+   * collision happen. The side is not asked for: the train is already coming from one.
+   */
+  | { type: 'mainline.redFlag'; flag: boolean; cardId?: CardId }
   /**
    * Flying Switch — cut cars off behind the engine and roll them into an adjacent industry, without
    * the engine entering it.
@@ -312,7 +326,11 @@ export type RejectionCode =
   /** §3.3 (Gitea#11) — this seat has already voted on this extension. */
   | 'ALREADY_VOTED'
   /** §11 (Gitea#5) — answering a Yard Office offer that is not open. */
-  | 'NO_YARD_OFFICE_OFFER';
+  | 'NO_YARD_OFFICE_OFFER'
+  /** §Q (Gitea#19) — answering a Red Flag prompt that is not open. */
+  | 'NO_RED_FLAG_PROMPT'
+  /** §Q (Gitea#19) — this district already has a flag on that side. */
+  | 'ALREADY_FLAGGED';
 
 export type Rejection = { code: RejectionCode; message: string };