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.

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 Martino Ferrari
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.