refactor: migrate to bun test, add integration tests, reorganize sources

- Replace jest with bun test
- Remove meaningless mock tests (main.test.ts, __fixtures__/)
- Add full pipeline integration test (physics + font + SVG)
- Move source files from src/server/ to src/ root
- Move font to public/ directory
- Remove rollup-plugin-copy dependency
- Update rollup config and jest config
- Update dist bundle

Author: flycran

__fixtures__/core.ts

@@ -0,0 +1,116 @@
+/**
+ * Integration test: full pipeline from gensvg → physics → font loading → SVG output.
+ */
+import { afterEach, beforeEach, describe, expect, it, mock } from 'bun:test'
+
+function mockContributionsResponse() {
+  return {
+    total: 7,
+    contributions: [
+      [
+        { date: '2026-07-19', intensity: 1, count: 1 },
+        { date: '2026-07-20', intensity: 2, count: 5 },
+        { date: '2026-07-21', intensity: 3, count: 10 },
+        { date: '2026-07-22', intensity: 0, count: 0 },
+        { date: '2026-07-23', intensity: 4, count: 20 },
+        { date: '2026-07-24', intensity: 1, count: 3 },
+        { date: '2026-07-25', intensity: 2, count: 7 }
+      ]
+    ]
+  }
+}
+
+const mockFetchContributions = mock(async () => mockContributionsResponse())
+mock.module('../src/github-contributions.ts', () => ({
+  fetchContributions: mockFetchContributions,
+  INTENSITY_COLORS: ['#ebedf0', '#9be9a8', '#40c463', '#30a14e', '#216e39']
+}))
+
+const { gensvg } = await import('../src/gensvg.js')
+
+describe('full pipeline integration', () => {
+  beforeEach(() => {
+    mockFetchContributions.mockImplementation(async () =>
+      mockContributionsResponse()
+    )
+  })
+
+  afterEach(() => {
+    mockFetchContributions.mockReset()
+  })
+
+  it('generates a valid SVG with contributions and text paths', async () => {
+    const svg = await gensvg({
+      username: 'testuser',
+      text: 'Hi',
+      fontSize: 40,
+      sampleRate: 4,
+      shape: 'circle'
+    })
+
+    expect(typeof svg).toBe('string')
+    expect(svg).toContain('<svg')
+    expect(svg).toContain('xmlns="http://www.w3.org/2000/svg"')
+    expect(svg).toContain('</svg>')
+    expect(svg).toContain('<circle')
+    expect(svg).toContain('animateTransform')
+    expect(svg).toContain('<path d="')
+  })
+
+  it('generates square-shaped contributions when shape is square', async () => {
+    const svg = await gensvg({
+      username: 'testuser',
+      text: 'Hi',
+      fontSize: 40,
+      shape: 'square'
+    })
+
+    expect(svg).toContain('<rect')
+    expect(svg).not.toContain('<circle')
+  })
+
+  it('handles empty text gracefully', async () => {
+    const svg = await gensvg({
+      username: 'testuser',
+      text: '',
+      fontSize: 40
+    })
+
+    expect(svg).toContain('<svg')
+    expect(svg).toContain('</svg')
+  })
+
+  it('throws on invalid username (API error)', async () => {
+    mockFetchContributions.mockImplementation(async () => {
+      throw new Error(
+        'Failed to fetch contributions for "nonexistent": 404 Not Found'
+      )
+    })
+
+    await expect(
+      gensvg({ username: 'nonexistent', text: 'Test' })
+    ).rejects.toThrow('Failed to fetch contributions')
+  })
+
+  it('produces deterministic SVG for same inputs', async () => {
+    const svg1 = await gensvg({
+      username: 'testuser',
+      text: 'OK',
+      fontSize: 30,
+      shape: 'circle'
+    })
+
+    mockFetchContributions.mockImplementation(async () =>
+      mockContributionsResponse()
+    )
+
+    const svg2 = await gensvg({
+      username: 'testuser',
+      text: 'OK',
+      fontSize: 30,
+      shape: 'circle'
+    })
+
+    expect(svg1).toBe(svg2)
+  })
+})