feat(L06): add Git branch switch and merge card

tests/card_contract.test.mjs

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+import assert from 'node:assert/strict'; import { access, readFile } from 'node:fs/promises'; import test from 'node:test';
+const source=JSON.parse(await readFile(new URL('../json/card_source.json',import.meta.url),'utf8'));
+test('L06 identity and task order are stable',()=>{assert.equal(source.card.number,'06');assert.equal(source.card.uuid,'ee873c5e-c3c4-550f-a081-eac409df99cb');assert.deepEqual(source.tasks_order,['task01','task02','task03']);});
+test('branch, divergence and merge evidence are explicit',()=>{const text=JSON.stringify(source);for(const token of ['branch','switch','merge-base','rodzic','trace.log'])assert.match(text,new RegExp(token,'i'));});
+test('all task sources exist',async()=>{for(const name of ['task01_create_and_switch','task02_divergent_work','task03_merge_completed_work'])await access(new URL(`../src/tasks/${name}.sh`,import.meta.url));});

tests/card_state_db.test.mjs

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+import assert from 'node:assert/strict';
+import { mkdtemp, rm } from 'node:fs/promises';
+import os from 'node:os';
+import path from 'node:path';
+import test from 'node:test';
+import { CardStateDatabase } from '../scripts/lib/card_state_db.mjs';
+
+test('card state survives closing and reopening SQLite', async () => {
+  const directory = await mkdtemp(path.join(os.tmpdir(), 'stem-card-state-'));
+  const file = path.join(directory, 'state.sqlite3');
+  const state = {
+    schema: 'stem-card-navigation.v2',
+    revision: 7,
+    selected_at: '2026-07-17T12:00:00.000Z',
+    snapshot_ref: 'task04.alloc5.cursor'
+  };
+
+  try {
+    const first = new CardStateDatabase(file);
+    first.write('navigation', state);
+    first.close();
+
+    const second = new CardStateDatabase(file);
+    assert.deepEqual(second.read('navigation'), state);
+    assert.equal(second.read('missing'), null);
+    second.close();
+  } finally {
+    await rm(directory, { recursive: true, force: true });
+  }
+});
+
+test('audit log is append-only, ordered and keeps navigation timestamps', async () => {
+  const directory = await mkdtemp(path.join(os.tmpdir(), 'stem-card-audit-'));
+  const file = path.join(directory, 'state.sqlite3');
+  try {
+    const database = new CardStateDatabase(file);
+    const navigation = {
+      task: { id: 'task04' },
+      block: { id: 'allocator-flow' },
+      phase: { id: 'alloc5' },
+      step: { id: 'alloc5-commit' },
+      snapshot_ref: 'task04.alloc5.commit'
+    };
+    const first = database.appendEvent({
+      eventType: 'navigation.activate',
+      actor: 'nvim',
+      navigation,
+      occurredAt: '2026-07-17T12:00:00.000Z',
+      payload: { source: 'F2' }
+    });
+    const second = database.appendEvent({
+      eventType: 'checkpoint.ready',
+      actor: 'nvim',
+      navigation,
+      occurredAt: '2026-07-17T12:00:01.000Z'
+    });
+    assert.equal(second.id, first.id + 1);
+    assert.deepEqual(
+      database.listEvents({ afterId: first.id, limit: 10 }).map(event => event.event_type),
+      ['checkpoint.ready']
+    );
+    assert.equal(database.listEvents()[0].snapshot_ref, navigation.snapshot_ref);
+    database.close();
+  } finally {
+    await rm(directory, { recursive: true, force: true });
+  }
+});

tests/test_tasks.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+root="${CARD_ROOT:-$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/.." && pwd)}"; task="${1:?task required}"
+case "$task" in task01_create_and_switch|task02_divergent_work|task03_merge_completed_work) ;; *) exit 2 ;; esac
+exec env CARD_ROOT="$root" "$root/src/tasks/$task.sh"