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-lexer/src/lexer.rs

@@ -347,6 +347,10 @@ fn keyword_or_ident(s: String) -> Token {
         "else"     => Token::Else,
         "for"      => Token::For,
         "in"       => Token::In,
+        "test"     => Token::Test,
+        "seed"     => Token::Seed,
+        "assert"   => Token::Assert,
+        "target"   => Token::Target,
         "true"     => Token::BoolLiteral(true),
         "false"    => Token::BoolLiteral(false),
         _          => Token::Ident(s),

crates/el-lexer/src/token.rs

@@ -73,6 +73,14 @@ pub enum Token {
     For,
     /// `in`
     In,
+    /// `test` — test block definition
+    Test,
+    /// `seed` — graph seeding statement inside a test
+    Seed,
+    /// `assert` — assertion statement inside a test
+    Assert,
+    /// `target` — test target annotation (`target: e2e`)
+    Target,
     /// `true` / `false`
     BoolLiteral(bool),
 
@@ -163,6 +171,10 @@ impl std::fmt::Display for Token {
             Token::Else => write!(f, "else"),
             Token::For => write!(f, "for"),
             Token::In => write!(f, "in"),
+            Token::Test => write!(f, "test"),
+            Token::Seed => write!(f, "seed"),
+            Token::Assert => write!(f, "assert"),
+            Token::Target => write!(f, "target"),
             Token::BoolLiteral(b) => write!(f, "{b}"),
             Token::IntLiteral(n) => write!(f, "{n}"),
             Token::FloatLiteral(n) => write!(f, "{n}"),