Add comprehensive unit tests and project documentation

[Copilot]

Adds 117 unit tests and project documentation to establish baseline test coverage across core packages.

Test Coverage Added

Core (draw2d) - 82.0% coverage

• matrix_test.go (26 tests): affine transformations, composition, inversion
• path_test.go (19 tests): path construction, curves, arcs, subpaths

Base (draw2dbase) - 76.6% coverage

• stack_gc_test.go (27 tests): GraphicContext state, transforms, save/restore
• dasher_test.go (6 tests): dash patterns, state preservation
• flattener_test.go (9 tests): path flattening, transforms
• stroker_test.go (4 tests): line stroking, width handling

Backends

• draw2dkit (10 tests): shapes, stroke/fill operations - 100% coverage
• draw2dimg (16 tests): RGBA rendering, DPI, clear operations - 58.8% coverage

Documentation

.github/copilot-instructions.md documents:

• Code style (headers, comments, naming conventions)
• Test patterns (table-driven tests, t.TempDir(), issue references)
• Architecture (backend structure, GraphicContext interface)
• Coverage goals

All tests use standard testing package, no external dependencies.

Original prompt

Objective

Add comprehensive unit tests and a .github/copilot-instructions.md file to the draw2d project.

Part 1: Copilot Instructions File

Create .github/copilot-instructions.md with the following content exactly:

# Copilot Instructions for draw2d

## Project Overview

draw2d is a Go 2D vector graphics library with multiple backends:
- `draw2d` — Core package: interfaces (`GraphicContext`, `PathBuilder`), types (`Matrix`, `Path`), and font management
- `draw2dbase` — Base implementations shared across backends (`StackGraphicContext`, flattener, stroker, dasher)
- `draw2dimg` — Raster image backend (using freetype-go)
- `draw2dpdf` — PDF backend (using gofpdf)
- `draw2dsvg` — SVG backend
- `draw2dgl` — OpenGL backend
- `draw2dkit` — Drawing helpers (`Rectangle`, `Circle`, `Ellipse`, `RoundedRectangle`)
- `samples/` — Example drawings used as integration tests

## Language and Conventions

- All code, comments, commit messages, and documentation must be written in **English**.
- The project uses **Go 1.20+** (see `go.mod`).

## Code Style

### File Headers

Source files include a copyright header and creation date:
```go
// Copyright 2010 The draw2d Authors. All rights reserved.
// created: 21/11/2010 by Laurent Le Goff

New files should follow the same pattern with the current date and author name.

Comments

• Exported types, functions, and methods must have GoDoc comments.
• Comments should start with the name of the thing being documented:

  // Rectangle draws a rectangle using a path between (x1,y1) and (x2,y2)
  func Rectangle(path draw2d.PathBuilder, x1, y1, x2, y2 float64) {

• Package comments go in the main source file or a doc.go file.

Naming

• Follow standard Go naming conventions (camelCase for unexported, PascalCase for exported).
• Backend packages are named draw2d<backend> (e.g., draw2dimg, draw2dpdf).
• The GraphicContext struct in each backend embeds *draw2dbase.StackGraphicContext.

Error Handling

• Functions that can fail return error as the last return value.
• Do not silently ignore errors — log or return them.

Testing

Structure

• Unit tests go alongside the source file they test (e.g., matrix_test.go tests matrix.go).
• Integration/sample tests live in samples_test.go, draw2dpdf/samples_test.go, etc.
• Test output files go in the output/ directory (generated, not committed).

Writing Tests

• Use the standard testing package only — no external test frameworks.
• Use table-driven tests where multiple inputs share the same logic:

  tests := []struct {
      name string
      // ...
  }{
      {"case1", ...},
      {"case2", ...},
  }
  for _, tt := range tests {
      t.Run(tt.name, func(t *testing.T) { ... })
  }

• Tests must not depend on external resources (fonts, network) unless testing that specific integration.
• For image-based tests, use image.NewRGBA(image.Rect(0, 0, w, h)) as the canvas.
• Use t.TempDir() for any file output in tests.
• Reference GitHub issue numbers in regression test comments:

  // Test related to issue #95: DashVertexConverter state preservation

Running Tests

go test ./...
go test -cover ./... | grep -v "no test"

Test Coverage Goals

• Every exported function and method should have at least one unit test.
• Core types (Matrix, Path, StackGraphicContext) should have thorough coverage.
• Backend-specific operations (Stroke, Fill, FillStroke, Clear) should verify pixel output where possible.
• Known bugs in the issue tracker should have corresponding regression tests.

Documentation

• When adding or changing public API, update the GoDoc comments accordingly.
• When fixing a bug, add a comment referencing the issue number.
• If a change affects behavior described in README.md or package READMEs, update them.
• The samples/ directory serves as living documentation — keep samples working after changes.

Architecture Notes

• All backends implement the draw2d.GraphicContext interface defined in gc.go.
• draw2dbase.StackGraphicContext provides the common state management (colors, transforms, font, path). Backends embed it and override rendering methods (Stroke, Fill, FillStroke, string drawing, etc.).
• The draw2dkit helpers operate on draw2d.PathBuilder, not GraphicContext, making them backend-agnostic.
• Matrix is a [6]float64 affine transformation matrix. Coordinate system follows the HTML Canvas 2D Context conventions.

## Part 2: Unit Tests

Add the following test files. Do NOT modify any existing files. All tests must pass with `go test ./...`.

### 2.1 `matrix_test.go` (root package `draw2d`)

Unit tests for `Matrix` type in `matrix.go`. Tests to include:

- `TestNewIdentityMatrix` — verify `IsIdentity()` returns true, equals `Matrix{1, 0, 0, 1, 0, 0}`
- `TestNewTranslationMatrix` — verify `GetTranslation()` returns correct values, `IsTranslation()`...

</details>



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