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-compiler/src/error.rs

@@ -0,0 +1,29 @@
+//! Compiler error type.
+
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum CompileError {
+    #[error("lex error: {0}")]
+    Lex(#[from] el_lexer::LexError),
+
+    #[error("parse error: {0}")]
+    Parse(#[from] el_parser::ParseError),
+
+    #[error("type error: {0}")]
+    Type(String),
+
+    #[error("codegen error: {0}")]
+    Codegen(String),
+
+    #[error("seal error: {0}")]
+    Seal(#[from] el_seal::SealError),
+
+    #[error("serialization error: {0}")]
+    Serialization(String),
+
+    #[error("io error: {0}")]
+    Io(String),
+}
+
+pub type CompileResult<T> = Result<T, CompileError>;