fix(text): carry a glyph's negative bearing and harden the bake's extents

tests/CMakeLists.txt

@@ -91,6 +91,16 @@ if(TARGET plot::font_ttf)
   target_compile_definitions(plot_tests PRIVATE
     PLOT_CORE_ARCHIVE="$<TARGET_FILE:plot_core>"
     PLOT_CMAKE_COMMAND="${CMAKE_COMMAND}")
+  # The cases compare a bake against the rasteriser's own glyph bitmaps, which is
+  # the only reference that can tell a complete cell from a clipped one, and for
+  # that the test needs the vendored header's *declarations* (its implementation is
+  # already in the binary through plot::font_ttf). This is the optional target's
+  # own test, compiled only when that target exists, and it is the one target
+  # outside src/text that sees third_party/: the isolation this project promises --
+  # plot_core's archive and every public header under include/plot/ -- is what
+  # plot_core_stays_free_of_stb_truetype keeps asserting, and it is unaffected by
+  # which include paths a test binary has.
+  target_include_directories(plot_tests PRIVATE "${CMAKE_SOURCE_DIR}/third_party")
 endif()
 
 # Same switches the library is built with: the cost tests read the counters