Add PostgreSQL virtual table extension and SQLite utilities

- Implement pg_vtab as a minimal SQLite virtual table module backed by PostgreSQL.
- Create sqlite2pgsql utility for copying tables from SQLite to PostgreSQL.
- Add sqlite_pg_plugin to expose PostgreSQL tables to SQLite.
- Include Makefile for building the pg_vtab shared object.
- Update README with usage instructions and limitations.
- Remove obsolete cache.zip file.

Makefile

@@ -0,0 +1,15 @@
+CC = gcc
+CFLAGS = -fPIC -O2 -I/usr/include
+LDFLAGS = -shared
+LIBS = -lpq
+
+TARGET = pg_vtab.so
+SRC = pg_vtab.c
+
+all: $(TARGET)
+
+$(TARGET): $(SRC)
+	$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^ $(LIBS)
+
+clean:
+	rm -f $(TARGET)