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-compiler/src/codegen.rs

@@ -121,6 +121,9 @@ impl Codegen {
             Stmt::TypeDef { .. } | Stmt::EnumDef { .. } => {
                 // Type and enum definitions are compile-time only; no runtime code.
             }
+            // Test-related statements — skipped during normal compilation.
+            // The el-test crate walks the AST directly rather than running compiled bytecode.
+            Stmt::TestDef { .. } | Stmt::Seed(..) | Stmt::Assert(..) => {}
         }
         Ok(())
     }
@@ -327,7 +330,10 @@ fn stmt_span(stmt: &Stmt) -> el_lexer::Span {
         | Stmt::Expr(_, span)
         | Stmt::FnDef { span, .. }
         | Stmt::TypeDef { span, .. }
-        | Stmt::EnumDef { span, .. } => *span,
+        | Stmt::EnumDef { span, .. }
+        | Stmt::TestDef { span, .. }
+        | Stmt::Seed(_, span)
+        | Stmt::Assert(_, span) => *span,
     }
 }