feat: project skeleton, core types and math with transform tests

docs/superpowers/plans/2026-09-27-plot-library-scaffold.md

@@ -247,8 +247,11 @@ Implement `from_range` and `log10_positive` in `src/math/transform.cpp`. `from_r
 ```cmake
 add_executable(plot_tests main.cpp test_transform.cpp)
 target_link_libraries(plot_tests PRIVATE plot::core)
-add_test(NAME transform COMMAND plot_tests)
+add_test(NAME plot_tests COMMAND plot_tests)
 ```
+Every later task adds its new `test_*.cpp` to the `plot_tests` source list; there is
+exactly one `add_test` entry, because the harness runs every registered case in one
+binary. The top-level `CMakeLists.txt` calls `enable_testing()` when `PLOT_BUILD_TESTS`.
 
 Top-level `CMakeLists.txt`: `cmake_minimum_required(VERSION 3.24)`, `project(plot VERSION 0.1.0 LANGUAGES CXX)`, include `cmake/plot_options.cmake`, declare the six `PLOT_BUILD_*` options from the plan header, build `plot_core` from `src/**/*.cpp` (explicit list), `add_library(plot::core ALIAS plot_core)`, `target_include_directories(plot_core PUBLIC include)`, `target_compile_features(plot_core PUBLIC cxx_std_20)`, `target_link_libraries(plot_core PRIVATE ${CMAKE_DL_LIBS})`, then `add_subdirectory(tests)` when `PLOT_BUILD_TESTS`.
 
@@ -688,7 +691,7 @@ Implementation rules for `src/data/minmax_pyramid.cpp`:
 
 `StreamSeries`: `RingBuffer<f32>` of rounded-up capacity, atomic `produced_`, `f64 x0_`, `f64 dx_`; `push` is the only producer entry point and MUST NOT touch the pyramid; `acquire()` reads `produced_` once, grows the pyramid by the newly produced samples, and updates bounds. `x_bounds()` derives from the oldest retained index and `dx_`, so a wrap shifts the window forward.
 
-Register the new tests in `tests/CMakeLists.txt` with `add_test(NAME ring_buffer COMMAND plot_tests)` etc. (the harness runs all cases in one binary; the multiple `add_test` names just label subsets in ctest output).
+Add `test_ring_buffer.cpp`, `test_minmax_pyramid.cpp` and `test_series.cpp` to the existing `plot_tests` target's source list in `tests/CMakeLists.txt`. Do not add new `add_test` entries — one `add_test(NAME plot_tests ...)` runs every registered case.
 
 - [ ] **Step 4: Run tests to verify they pass**