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>

tests/shared/transponder.test.ts

@@ -0,0 +1,49 @@
+import { test } from 'node:test'
+import assert from 'node:assert/strict'
+import { RESERVED_SQUAWKS, generateSquawk, isValidSquawk } from '../../shared/utils/transponder.ts'
+
+// Enough draws that a decimal generator (which would emit 8/9 roughly a quarter
+// of the time per digit) could not pass by chance.
+const DRAWS = 5000
+
+test('generateSquawk only ever emits octal digits', () => {
+  for (let i = 0; i < DRAWS; i++) {
+    const code = generateSquawk()
+    assert.match(code, /^[0-7]{4}$/, `non-octal squawk generated: ${code}`)
+  }
+})
+
+test('generateSquawk never emits a reserved code', () => {
+  for (let i = 0; i < DRAWS; i++) {
+    const code = generateSquawk()
+    assert.ok(!RESERVED_SQUAWKS.has(code), `reserved squawk generated: ${code}`)
+  }
+})
+
+test('generateSquawk covers the code space rather than returning a constant', () => {
+  const seen = new Set<string>()
+  for (let i = 0; i < DRAWS; i++) seen.add(generateSquawk())
+  assert.ok(seen.size > 1000, `expected a spread of codes, got ${seen.size} distinct`)
+})
+
+test('isValidSquawk rejects the digits a transponder cannot display', () => {
+  assert.equal(isValidSquawk('2891'), false)
+  assert.equal(isValidSquawk('4592'), false)
+  assert.equal(isValidSquawk('1234'), true)
+})
+
+test('isValidSquawk rejects reserved codes', () => {
+  assert.equal(isValidSquawk('7500'), false)
+  assert.equal(isValidSquawk('7600'), false)
+  assert.equal(isValidSquawk('7700'), false)
+  assert.equal(isValidSquawk('7000'), false)
+  assert.equal(isValidSquawk('2000'), false)
+  assert.equal(isValidSquawk('0000'), false)
+})
+
+test('isValidSquawk rejects malformed input', () => {
+  assert.equal(isValidSquawk(''), false)
+  assert.equal(isValidSquawk('123'), false)
+  assert.equal(isValidSquawk('12345'), false)
+  assert.equal(isValidSquawk('12A4'), false)
+})