feat: add PubGrub version-constraint solver

Implements Natalie Weizenbaum's PubGrub algorithm (Dart pub, 2018)
over semver.h.  Given a set of (package, version, dependencies)
facts plus root dependencies, the solver picks one version per
required package that satisfies every accumulated constraint, or
reports that no such assignment exists.

Lifecycle follows the value-type idiom used by sbuf: caller-owned
header struct, opaque state pointer, init/release pairs.  The
component manager wires this in through cmd/idf/component/solve.c.

Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>

Author: fhrbata

Makefile

@@ -190,6 +190,7 @@ LIB_SRCS := \
 	binary.c \
 	elf2image.c \
 	progress.c \
+	pubgrub.c \
 	sbuf.c \
 	semver.c \
 	slip.c \