Fix Makefiles and add README

The http/ and hashtable/ Makefiles named sources that do not exist
(test.c, net.c, net.h in http/; http.h in hashtable/) and had no way to
find the sibling hashtable/llist sources they link against, so neither
directory built at all. Both now compile those sources with explicit
recipes.

VPATH would be the terser fix but is deliberately avoided: it searches
for targets as well as sources, so a stale ../llist/test.o gets linked
in place of a freshly compiled one, and hashtable/test silently runs the
linked-list suite instead of its own.

llist/Makefile built fine; its .PHONY read "all," with a comma, making
that the literal target name, and test.o was missing its llist.h
dependency.

README covers layout, build, run, the use()/Handler routing API, and the
library tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

llist/Makefile

@@ -6,14 +6,12 @@ OBJS=test.o llist.o
 all: test
 
 test: $(OBJS)
-	gcc -o $@ $^
-
-test.o: test.c
+	$(CC) -o $@ $^
 
+test.o: test.c llist.h
 llist.o: llist.c llist.h
 
 clean:
-	rm -f $(OBJS)
-	rm -f test
+	rm -f $(OBJS) test
 
-.PHONY: all, clean
+.PHONY: all clean