add el-tests vessel: manifest-based test suite under tests/suite/

Restructures the test suite as a proper El vessel with manifest.el and
src/ layout, eliminating the bash run.sh harness. CI runs the suite with
two commands: `cd tests/suite && elb && ./dist/el-tests`. Exit code is
the fail count (0 = all pass).

163 test cases across 7 modules: string (52), math (13), json (26),
state (11), time (25), fs (16), collections (19).

tests/suite/manifest.el

@@ -0,0 +1,17 @@
+// tests/suite/manifest.el — El test suite vessel manifest.
+//
+// Build and run:
+//   cd tests/suite && elb && ./dist/el-tests
+//
+// Exit code equals the number of failing assertions (0 = all pass).
+
+package "el-tests" {
+    version "0.1.0"
+    description "El runtime test suite"
+    edition "2026"
+}
+
+build {
+    entry "src/test_all.el"
+    output "dist/"
+}