fix(clearance): generate octal squawks and skip reserved codes

genSquawk() drew a decimal number in 1000-8999, so clearances could contain
the digits 8 or 9 — codes no transponder can dial. Replace it with a shared
generateSquawk() that draws four octal digits and re-rolls the reserved codes
(7500/7600/7700, 7000, 2000, 1200, 0000).

shared/learn/scenario.ts had its own octal generator that could still draw an
emergency code; it now uses the same helper.

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

shared/utils/communicationsEngine.ts

@@ -9,6 +9,7 @@ import type {
 } from '../types/decision'
 import type { FlowActivationInstruction, FlowActivationMode, LLMDecisionTrace } from '../types/llm'
 import { normalizeRadioPhrase, DEFAULT_AIRLINE_TELEPHONY } from './radioSpeech'
+import { generateSquawk } from './transponder'
 
 // --- DecisionTree runtime types ---
 type Role = 'pilot' | 'atc' | 'system'
@@ -783,7 +784,7 @@ export default function useCommunicationsEngine() {
             dest: fpl.arr || fpl.arrival || 'EDDM',
             stand: genStand(),
             runway: genRunway(),
-            squawk: fpl.assignedsquawk || genSquawk(),
+            squawk: fpl.assignedsquawk || generateSquawk(),
             atis_code: genATIS(),
             sid: genSID(fpl.route || ''),
             transition: 'DCT',
@@ -1509,10 +1510,6 @@ export default function useCommunicationsEngine() {
         return arr[Math.floor(Math.random() * arr.length)]
     }
 
-    function genSquawk() {
-        return String(Math.floor(Math.random() * 8000 + 1000)).padStart(4, '0')
-    }
-
     function genATIS() {
         const letters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
         return letters[Math.floor(Math.random() * letters.length)]

shared/utils/transponder.ts

@@ -0,0 +1,39 @@
+/**
+ * Transponder (SSR) code generation.
+ *
+ * A squawk is an *octal* four-digit code — every digit is 0–7, because each
+ * digit is encoded in three pulse positions. A code containing 8 or 9 cannot be
+ * dialled into a real transponder, so it must never reach a clearance.
+ */
+
+/**
+ * Codes ATC will not assign as a discrete code.
+ *
+ * 7500/7600/7700 are the ICAO emergency codes (unlawful interference, radio
+ * failure, general emergency); 7000 is the European VFR conspicuity code and
+ * 1200 its US equivalent; 2000 is the code for entering from a non-SSR area;
+ * 0000 is a non-code used to indicate a transponder fault.
+ */
+export const RESERVED_SQUAWKS = new Set([
+    '7500', '7600', '7700', '7000', '2000', '1200', '0000',
+])
+
+/** True when `code` is a well-formed, assignable discrete squawk. */
+export function isValidSquawk(code: string): boolean {
+    if (!/^[0-7]{4}$/.test(code)) return false
+    return !RESERVED_SQUAWKS.has(code)
+}
+
+/** A random assignable discrete squawk: four octal digits, no reserved code. */
+export function generateSquawk(): string {
+    // The reserved set is tiny next to the 4096-code space, so re-rolling
+    // terminates immediately in practice; the bound just makes that guaranteed.
+    for (let attempt = 0; attempt < 20; attempt++) {
+        let code = ''
+        for (let digit = 0; digit < 4; digit++) {
+            code += String(Math.floor(Math.random() * 8))
+        }
+        if (isValidSquawk(code)) return code
+    }
+    return '1000'
+}