test(observability): close 2 MAJOR test gaps from SKAL-2 review

F1 cache-analyzer regression guard (SC5): pin percentile (wall_time_ms_p50/p90)
and time-range (oldest/newest_event_iso) — 2 of 3 'unchanged' categories were
previously un-asserted on the mixed-input fixture.

F2 lastMainChainModel: add direct coverage — last-wins across 2 distinct
main-chain models, sidechain exclusion (even when the sidechain is the last
record), and model-absent → null propagating to deriveCost refuse-to-estimate.

804 -> 807 tests (805 pass / 0 fail / 2 skipped). No test pins the test count.

tests/lib/token-usage.test.mjs

@@ -16,6 +16,7 @@ import {
   deriveCost,
   buildRecord,
   upsertSessionRecord,
+  lastMainChainModel,
   PRICE_TABLE,
   PRICE_TABLE_VERSION,
 } from '../../lib/stats/token-usage.mjs';
@@ -140,3 +141,39 @@ test('upsertSessionRecord — empty file → single record, trailing newline', (
   assert.ok(out.endsWith('\n'));
   assert.deepEqual(out.trim().split('\n').map(JSON.parse).length, 1);
 });
+
+// ---- lastMainChainModel ----------------------------------------------------
+
+test('lastMainChainModel — last main-chain model wins; sidechain excluded even when last', () => {
+  // Two distinct main-chain models (opus then sonnet) → sonnet (last) wins.
+  // A sidechain record sits AFTER sonnet: if exclusion broke it would win;
+  // if last-wins broke, opus (first) would win. Correct answer pins both.
+  const text = [
+    JSON.stringify({ type: 'assistant', isSidechain: false, message: { model: 'claude-opus-4-8', usage: {} } }),
+    JSON.stringify({ type: 'assistant', isSidechain: false, message: { model: 'claude-sonnet-4-6', usage: {} } }),
+    JSON.stringify({ type: 'assistant', isSidechain: true, message: { model: 'claude-sidechain-ZZZ', usage: {} } }),
+    JSON.stringify({ type: 'user', message: { content: 'ignored' } }),
+  ].join('\n');
+  assert.equal(lastMainChainModel(text), 'claude-sonnet-4-6');
+});
+
+test('lastMainChainModel — no main-chain model → null → deriveCost refuses to estimate', () => {
+  // Only a sidechain model, a user line, and a malformed line: no main-chain model.
+  const text = [
+    JSON.stringify({ type: 'assistant', isSidechain: true, message: { model: 'claude-opus-4-8', usage: {} } }),
+    JSON.stringify({ type: 'user', message: { content: 'hi' } }),
+    'not json',
+  ].join('\n');
+  const model = lastMainChainModel(text);
+  assert.equal(model, null);
+  // null model must propagate to refuse-to-estimate (honesty contract).
+  const { cost_usd, is_estimate } = deriveCost({ tokens_input: 1000 }, model);
+  assert.equal(cost_usd, null);
+  assert.equal(is_estimate, true);
+});
+
+test('lastMainChainModel — empty / nullish input → null', () => {
+  assert.equal(lastMainChainModel(''), null);
+  assert.equal(lastMainChainModel(null), null);
+  assert.equal(lastMainChainModel(undefined), null);
+});