rename crates/ to engrams/; add el-compiler el package with bootstrap artifact

- crates/ → engrams/ (Rust engrams live here)
- el-compiler/ added: el self-hosting compiler as an el package
  - src/{compiler,lexer,parser,codegen}.el
  - bootstrap/el-compiler.elc (114KB, Rust-compiled seed)
- el.toml Cargo.toml workspace paths updated
- neuron-rs cross-repo path deps fixed (were pointing to products/ instead of foundation/)

engrams/el-arch/src/error.rs

@@ -0,0 +1,45 @@
+//! Diagnostic types for the architectural checker.
+
+/// Severity level of an architectural diagnostic.
+#[derive(Debug, Clone, PartialEq)]
+pub enum Severity {
+    Error,
+    Warning,
+}
+
+/// A single architectural diagnostic (error or warning).
+#[derive(Debug, Clone)]
+pub struct ArchDiagnostic {
+    pub severity: Severity,
+    /// Rule identifier, e.g. "VBD-001".
+    pub rule: String,
+    pub message: String,
+    /// Function name or other location hint.
+    pub location: Option<String>,
+}
+
+/// Type alias — an ArchError is an ArchDiagnostic with Severity::Error.
+pub type ArchError = ArchDiagnostic;
+
+/// Type alias — an ArchWarning is an ArchDiagnostic with Severity::Warning.
+pub type ArchWarning = ArchDiagnostic;
+
+impl ArchDiagnostic {
+    pub fn error(rule: impl Into<String>, message: impl Into<String>, location: Option<String>) -> Self {
+        Self {
+            severity: Severity::Error,
+            rule: rule.into(),
+            message: message.into(),
+            location,
+        }
+    }
+
+    pub fn warning(rule: impl Into<String>, message: impl Into<String>, location: Option<String>) -> Self {
+        Self {
+            severity: Severity::Warning,
+            rule: rule.into(),
+            message: message.into(),
+            location,
+        }
+    }
+}