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-parser/src/ast.rs

@@ -2,6 +2,36 @@
 
 use el_lexer::Span;
 
+// ── Test-specific nodes ───────────────────────────────────────────────────────
+
+/// Which graph a test should execute against.
+#[derive(Debug, Clone, PartialEq)]
+pub enum TestTarget {
+    /// In-memory graph — default, zero external dependencies.
+    Unit,
+    /// Real Engram database pointed at by `ENGRAM_URL` / `ENGRAM_DB_PATH`.
+    E2e,
+    /// Run against both unit (in-memory) and e2e (real DB).
+    Both,
+}
+
+/// A `seed Node { ... }` or `seed Edge { ... }` statement inside a test block.
+#[derive(Debug, Clone, PartialEq)]
+pub enum SeedStmt {
+    Node {
+        node_type: String,
+        content: String,
+        importance: f32,
+        tier: Option<String>,
+    },
+    Edge {
+        from: String,
+        to: String,
+        relation: String,
+        weight: f32,
+    },
+}
+
 // ── Literals ──────────────────────────────────────────────────────────────────
 
 #[derive(Debug, Clone, PartialEq)]
@@ -144,6 +174,17 @@ pub enum Stmt {
         variants: Vec<Variant>,
         span: Span,
     },
+    /// `test "name" [target: unit|e2e|both] { body }`
+    TestDef {
+        name: String,
+        target: TestTarget,
+        body: Vec<Stmt>,
+        span: Span,
+    },
+    /// `seed Node { ... }` or `seed Edge { ... }`
+    Seed(SeedStmt, Span),
+    /// `assert <expr>`
+    Assert(Expr, Span),
 }
 
 // ── Top-level program ─────────────────────────────────────────────────────────