fix(calendar): preserve manual picks when regenerating a month (#2)

generatePlan gains a preAssigned parameter: manual entries occupy slots,
count toward eligibility (maxPerMonth/minGap) and are never double-booked;
only the remaining slots per day are auto-filled. The generate handler now
keeps manual:true entries for the month instead of wiping them.

Adds tests for slot occupation, fully-manual days, and maxPerMonth counting.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/algorithm.test.mjs

@@ -187,4 +187,31 @@ function emp(id, constraints = {}) {
   assert.deepEqual(result.skippedDates, ['2026-04-02'], 'Thu in skippedDates');
 }
 
+// preAssigned: manual pick occupies a slot, only remaining slots auto-filled, no double-book
+{
+  const employees = [emp('alice'), emp('bob'), emp('clara')];
+  const manual = [{ date: '2026-04-01', employeeId: 'alice', manual: true }];
+  const result = generatePlan(['2026-04-01'], employees, [], fixed, 2, manual);
+  assert.equal(result.assignments.length, 1, 'slotsPerDay=2 with 1 manual → 1 auto entry');
+  assert.notEqual(result.assignments[0].employeeId, 'alice', 'manual employee not double-booked');
+  assert.equal(result.assignments[0].manual, false, 'auto entry has manual:false');
+}
+
+// preAssigned: day fully staffed by manual entries → nothing generated, not skipped
+{
+  const employees = [emp('alice'), emp('bob')];
+  const manual = [{ date: '2026-04-01', employeeId: 'alice', manual: true }];
+  const result = generatePlan(['2026-04-01'], employees, [], fixed, 1, manual);
+  assert.equal(result.assignments.length, 0, 'fully manual day → no auto entries');
+  assert.deepEqual(result.skippedDates, [], 'fully manual day not skipped');
+}
+
+// preAssigned counts toward maxPerMonth eligibility
+{
+  const employees = [emp('alice', { maxPerMonth: 1 }), emp('bob'), emp('clara')];
+  const manual = [{ date: '2026-04-01', employeeId: 'alice', manual: true }];
+  const result = generatePlan(['2026-04-02'], employees, [], fixed, 1, manual);
+  assert.notEqual(result.assignments[0].employeeId, 'alice', 'manual entry counts toward maxPerMonth');
+}
+
 console.log('All algorithm tests passed.');