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-registry/src/error.rs

@@ -0,0 +1,39 @@
+//! Registry error types.
+
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum RegistryError {
+    #[error("http error: {0}")]
+    Http(#[from] reqwest::Error),
+
+    #[error("io error: {0}")]
+    Io(#[from] std::io::Error),
+
+    #[error("json error: {0}")]
+    Json(#[from] serde_json::Error),
+
+    #[error("no version of '{name}' satisfies '{req}'")]
+    NoMatchingVersion { name: String, req: String },
+
+    #[error("package '{0}' not found in registry")]
+    NotFound(String),
+
+    #[error("checksum mismatch for '{name}': expected {expected}, got {actual}")]
+    ChecksumMismatch {
+        name: String,
+        expected: String,
+        actual: String,
+    },
+
+    #[error("authentication required: provide an API key")]
+    AuthRequired,
+
+    #[error("registry returned error {status}: {message}")]
+    RegistryError { status: u16, message: String },
+
+    #[error("manifest error: {0}")]
+    Manifest(#[from] el_manifest::ManifestError),
+}
+
+pub type RegistryResult<T> = Result<T, RegistryError>;