feat: due_at parser and event upserter

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BCrwHHnGCB5XH968Nxokqw

core/tests/Unit/Ingest/DueAtParserTest.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace Tests\Unit\Ingest;
+
+use App\Enums\DueMode;
+use App\Ingest\DueAtParser;
+use InvalidArgumentException;
+use Tests\TestCase; // Laravel TestCase: Eloquent date-casts need the connection grammar even in memory
+
+class DueAtParserTest extends TestCase
+{
+    public function test_date_only_is_whole_day_local(): void
+    {
+        $p = DueAtParser::parse('2026-08-14');
+        $this->assertSame(DueMode::Local, $p->mode);
+        $this->assertSame('2026-08-14', $p->date);
+        $this->assertSame('24:00:00', $p->time);
+        $this->assertNull($p->instant);
+        $this->assertSame('2026-08-14', $p->dayFor('Asia/Shanghai'));
+    }
+
+    public function test_datetime_without_offset_is_local(): void
+    {
+        $p = DueAtParser::parse('2026-09-10T14:00');
+        $this->assertSame(DueMode::Local, $p->mode);
+        $this->assertSame('2026-09-10', $p->date);
+        $this->assertSame('14:00:00', $p->time);
+
+        $this->assertSame('14:05:30', DueAtParser::parse('2026-09-10T14:05:30')->time);
+    }
+
+    public function test_datetime_with_offset_is_fixed(): void
+    {
+        $p = DueAtParser::parse('2026-09-10T08:15+02:00');
+        $this->assertSame(DueMode::Fixed, $p->mode);
+        $this->assertNull($p->date);
+        $this->assertSame('2026-09-10T06:15:00+00:00', $p->instant->utc()->toIso8601String());
+        $this->assertSame('2026-09-10', $p->dayFor('America/Montevideo'));
+        $this->assertSame('2026-09-10', $p->dayFor('Asia/Shanghai'));
+
+        $this->assertSame(DueMode::Fixed, DueAtParser::parse('2026-09-10T06:15:00Z')->mode);
+    }
+
+    public function test_garbage_is_rejected(): void
+    {
+        foreach (['', 'tomorrow', '2026-13-01', '14:00', '2026-09-10 14:00', '2026-09-10T25:00'] as $bad) {
+            try {
+                DueAtParser::parse($bad);
+                $this->fail("accepted: $bad");
+            } catch (InvalidArgumentException) {
+                $this->addToAssertionCount(1);
+            }
+        }
+    }
+}