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.

README_pg_vtab.md

@@ -0,0 +1,38 @@
+# SQLite -> PostgreSQL virtual table (minimal)
+
+This directory contains a minimal SQLite extension that implements a read-only virtual table backed directly by PostgreSQL using libpq.
+
+Build
+-----
+You need `libpq` (Postgres client) and a C compiler. On Debian/Ubuntu:
+
+```bash
+sudo apt-get install libpq-dev build-essential
+make
+```
+
+This produces `pg_vtab.so`.
+
+Usage
+-----
+Start `sqlite3`, load the extension and create a virtual table pointing at a Postgres table:
+
+```sql
+.load ./pg_vtab.so
+CREATE VIRTUAL TABLE remote_tbl USING pg_vtab('host=localhost dbname=mydb user=me password=secret', 'public.mytable');
+SELECT * FROM remote_tbl LIMIT 10;
+```
+
+Limitations
+-----------
+- Minimal proof-of-concept: supports only full table scans.
+- All columns are presented as `TEXT`.
+- No constraint pushdown or type mapping sophistication.
+- Error handling and SQL injection hygiene are minimal; do not use on untrusted inputs without review.
+
+Next steps (optional)
+---------------------
+- Add support for WHERE constraints and indexed access (`xBestIndex`).
+- Map Postgres types more accurately to SQLite storage classes.
+- Implement server-side cursors for large tables.
+- Harden error reporting and resource cleanup.