feat: package manager, build system, native cross-compilation, plugin system

Add three new crates and extend the compiler and CLI toolchain:

- el-manifest: el.toml manifest parser using serde + toml crate; supports
  package info, registry/path/version deps, build config with seal key
  sources, cross targets, and plugins; Manifest::find_manifest() walks up
  the directory tree

- el-registry: HTTP registry client (reqwest + tokio) for
  packages.neurontechnologies.ai; PackageMetadata, fetch/download/publish/
  search, BLAKE3 checksum verification, local cache at ~/.engram/packages/

- el-build: build orchestrator with incremental builds (BLAKE3 file hashes
  in .el/build-cache.json), cross-compilation target tagging, dep resolution,
  plugin registry with on_ast/on_typed_ast/on_bytecode hooks, test runner,
  fmt/check/clean commands

- CrossTarget and NativeTarget enums with triple() and artifact_extension()
  methods; NativeTarget::Host detects compile-time platform via cfg! macros

- Plugin system: CompilerPlugin trait + PluginRegistry; dynamic loading is
  a marked TODO with clear extension point for libloading

- CLI extended with: new, add, remove, update, build --cross, run, test,
  check, fmt, clean, publish, search, plugin add/remove/list; old
  single-file commands moved to build-file/seal/unseal subcommands

- Fix pre-existing debugger.rs borrow error (unwrap_or temporary lifetime)
- Fix checker.rs and codegen.rs to handle TestDef/Seed/Assert Stmt variants
- Add spec/language.md sections 12-14: package system, build system,
  plugin system, cross-compilation targets table

130 tests passing, zero warnings

crates/el-types/src/checker.rs

@@ -150,6 +150,24 @@ impl TypeChecker {
             Stmt::TypeDef { .. } | Stmt::EnumDef { .. } => {
                 // Already handled in hoist pass
             }
+            Stmt::TestDef { body, .. } => {
+                // Type-check the test body statements
+                for s in body {
+                    self.check_stmt(s);
+                }
+            }
+            Stmt::Seed(_, _) => {
+                // Seed statements are data-seeding constructs; no type checking needed.
+            }
+            Stmt::Assert(expr, _) => {
+                let ty = self.infer_expr(expr);
+                if !self.env.check_compatible(&ty, &Type::Bool) {
+                    self.emit_error(TypeErrorKind::TypeMismatch {
+                        expected: "Bool".into(),
+                        got: ty.to_string(),
+                    });
+                }
+            }
         }
     }