feat(zeta): manifest model, validation, package.lua emitter

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <[email protected]>

testdata/golden/hello.package.lua

@@ -0,0 +1,9 @@
+return {
+  name    = "hello",
+  version = "1.0",
+  summary = "A tiny demonstration program",
+  url     = "https://example.org/packages/hello/hello-1.0.tar.gz",
+  sha256  = "731887527d1a72c57d1e64bad85cc341a4285354e773b2cd277420b744df2943",
+  deps    = {},
+  archive = { strip = 1 },
+}