Generalises the lv_conf.h fix, because it was never an LVGL problem. One
live compile resolved SIX config headers against the shared library dir in a
single build (sdkconfig.h, lv_conf.h, lv_conf_kconfig.h,
lv_rt_thread_conf.h, User_Config.h, zconf.h): six chances to merge a library
nobody asked for, and one of them did exactly that.
The rule is now about the shape of the name (*conf.h, *config.h, *_setup.h,
plus the known ones) and applies only to headers reached TRANSITIVELY -- a
config header the sketch includes itself is intent and still resolves.
Why skipping the lookup is safe rather than merely convenient: once a library
is merged its own headers are on the include path, so a config header shipped
BY the library that asks for it resolves at compile time with no lookup at
all. The lookup can only ever pull in a DIFFERENT library, which for a config
header is never what the project meant. A genuinely missing header now fails
where it should -- named, in the compiler output -- instead of inside the
sources of a library the sketch never mentioned.
Tests cover all six shapes, the direct-include exception and an ordinary
transitive dependency (Adafruit GFX -> BusIO) that must keep merging.
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