Add the Corporate Ladder pack, and two engine features it needed

Seventeen events across the mailroom: ordinary days, Marlene Hobb and Trevor
Pike, a misrouted envelope that can be delivered or kept, and a hard-times
branch that fires on negative money and always leaves an ungated way out.

Writing real content turned up two things the engine could not express.

Per-turn upkeep (`pack.upkeep`): the MVP spec wants recurring income and costs,
but effects only run when a player chooses something, and baking wages into
every option would be tedious and easy to get wrong. Upkeep entries carry their
own conditions, which turned out to be enough for the loan service charge to be
a three-step function that escalates with the size of the debt — no multiply
operation, no engine change. Upkeep changes are recorded separately from choice
changes so the UI can distinguish them.

Per-character starting relationships: Trevor starts below the pack default on
`likes_you` because he arrived two weeks earlier and has priced that in.
Characterisation as data rather than as an opening cutscene. Nobody starts
feared, either — `fears_you` defaults to 10, since a midpoint default would mean
every new hire arrives already intimidating.

The content suite plays 60 seeded runs and asserts every event fires and every
option is taken at least once. That caught real dead content: Trevor's favour
chain sat behind a gate no plausible player could reach, taking two events and a
hard-times escape route with it, and debt could pass 5,500 while the service
charge stayed flat, so the failure spiral flattened out instead of tightening.
Both fixed in the data — the gate softened and given another route up, the loan
capped and its charge tiered.

The validator also gained a real fix: it counted an event with a cooldown as a
stage's fallback, but a cooldown can exclude it, so a stage could still run dry.

86 tests. Reasoning in docs/DECISIONS.md §11-13, including the seeding trap that
makes a correlated chooser look exactly like a content bug.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VMSFHyVPitUoosW5wyEADj

test/validate.test.js

@@ -119,3 +119,25 @@ test('assertValidPack throws with every error listed', () => {
   assert.throws(() => assertValidPack(pack), /is invalid/);
   assert.doesNotThrow(() => assertValidPack(testPack));
 });
+
+test('upkeep is validated like any other effect list', () => {
+  const badPath = broken((pack) => {
+    pack.upkeep = [{ label: 'wage', effects: [{ path: 'monie', op: 'add', value: 20 }] }];
+  });
+  assert.match(messages(badPath), /monie.*not declared/s);
+
+  const empty = broken((pack) => { pack.upkeep = [{ label: 'wage', effects: [] }]; });
+  assert.match(messages(empty), /has no effects/);
+
+  const writesReserved = broken((pack) => {
+    pack.upkeep = [{ effects: [{ path: 'turn', op: 'add', value: 1 }] }];
+  });
+  assert.match(messages(writesReserved), /engine-owned state/);
+});
+
+test('a starting relationship for a stat that does not exist is caught', () => {
+  const result = broken((pack) => {
+    pack.characters[0].startingRelationship = { respects_you: 10 };
+  });
+  assert.match(messages(result), /unknown stat "respects_you"/);
+});