licence: MIT LICENSE and SPDX identifiers in every first-party file

The repository had no licence at all, so every source was all-rights-reserved
by default. Add the verbatim MIT text at LICENSE (Copyright (c) 2026 Martino
Ferrari) and a two-line SPDX header to each of the 113 first-party sources --
48 include/plot, 26 src, 10 adapters, 5 examples, 21 tests, 3 tools -- so the
grant travels with each file rather than relying on the root document alone.

Scripts keep their shebang on line 1; the SPDX lines follow it. Genuinely
vendored code is untouched: third_party/stb_truetype.h carries no header of
ours. src/render/embedded_mono_font.cpp is generated, so tools/fonts/gen_atlas.py
emit() now writes the two lines into its header comment and the file was
regenerated -- gen_atlas.py --check passes and the regeneration adds only those
comment lines to the previously committed file.

README's provenance section now states MIT and links LICENSE.

No code, test or public API change.

tests/egl_headless.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #include "egl_headless.hpp"
 
 #include <plot/gl/api.hpp>

tests/egl_headless.hpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #pragma once
 #include <plot/core/status.hpp>
 #include <plot/core/types.hpp>

tests/harness.hpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #pragma once
 #include <cstdio>
 #include <cmath>

tests/main.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #include "harness.hpp"
 
 int main() { return plottest::run_all(); }

tests/test_controller.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #include "harness.hpp"
 
 #include <plot/input/controller.hpp>

tests/test_figure.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 // `Figure`: several plots stacked in one column, sharing one x range.
 //
 // The layout cases pin the geometry the renderer draws with -- the row rects, the

tests/test_gl_context.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #include "harness.hpp"
 #include "egl_headless.hpp"
 

tests/test_glfw_host.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 // The GLFW host's event routing in figure mode: which row's controller a pointer pixel
 // reaches.
 //

tests/test_gtk_input.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 // The GTK adapter's input translation, driven directly.
 //
 // Four pieces of it are this project's logic and not a toolkit's, and none of them

tests/test_imgui_widget.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 // The Dear ImGui adapter's own logic, pinned against real ImGui frames on the
 // tests' surfaceless EGL context -- no window, no display server.
 //

tests/test_measure.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #include "harness.hpp"
 #include <plot/plot/measure.hpp>
 #include <plot/data/array_series.hpp>

tests/test_minmax_pyramid.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #include "harness.hpp"
 #include <plot/data/detail/cover_count.hpp>
 #include <plot/data/detail/merge_count.hpp>

tests/test_plot.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #include "harness.hpp"
 #include <plot/plot/plot.hpp>
 #include <plot/plot/detail/measure_count.hpp>

tests/test_primitives.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 // CPU-side contract of the render layer: the staging batches that feed the
 // instanced line and text pipelines, and the embedded font's metrics and atlas.
 // Nothing here touches GL, so it runs on any machine that can build the tests.

tests/test_qt_pointer.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 // The Qt adapter's pointer translation, driven directly. The widget's cached
 // pointer anchor is a plain value with no toolkit dependency (see
 // adapters/qt6/qt_pointer.hpp) precisely so the one thing a screenshot cannot show

tests/test_render_smoke.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 // The integration test: the whole stack on the real driver. A 1M-sample series,
 // the plot layer's layout/ticks/grid, the renderer's visible-range LOD selection
 // and both vertex-transfer strategies, all asserted by reading pixels back from

tests/test_ring_buffer.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #include "harness.hpp"
 #include <plot/data/ring_buffer.hpp>
 

tests/test_series.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #include "harness.hpp"
 #include <plot/data/array_series.hpp>
 #include <plot/data/detail/merge_count.hpp>

tests/test_ticks.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #include "harness.hpp"
 #include <plot/plot/ticks.hpp>
 #include <cmath>

tests/test_transform.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 #include "harness.hpp"
 
 #include <plot/math/transform.hpp>

tests/test_true_type_font.cpp

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
 // The optional runtime-font backend, on the real driver and on the real archive:
 // what a host gets when it points `plot::text::TrueTypeFont` at a font file.
 //