sqlite moves into the sx library: import vendors/sqlite/sqlite.sx

The amalgamation and the bindings now ship with sx itself
(sx library/vendors/sqlite/ — bindings + c/ amalgamation); every
import flips from ../src/db/sqlite.sx to vendors/sqlite/sqlite.sx,
resolved through the compiler's stdlib search paths. vendor/ and
src/db/ leave this repo entirely. make test 22/22 — the object cache
keys on content, not path, so the relocated source still hits the
existing cache entries.

tests/sqlite_api.sx

@@ -1,4 +1,4 @@
-// Pinned acceptance for the FULL SQLite mapping (src/db/sqlite.sx) —
+// Pinned acceptance for the FULL SQLite mapping (vendors/sqlite, shipped with sx) —
 // every wrapper family beyond the P5.1 smoke (which keeps owning the
 // vendored-version pin):
 //
@@ -22,7 +22,7 @@
 //     query answers match.
 #import "modules/std.sx";
 process :: #import "modules/std/process.sx";
-sq :: #import "../src/db/sqlite.sx";
+sq :: #import "vendors/sqlite/sqlite.sx";
 
 DBDIR :: ".sx-tmp/sqlite_api";