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-parser/src/error.rs

@@ -0,0 +1,54 @@
+//! Parser error types.
+
+use thiserror::Error;
+use el_lexer::{Span, Token};
+
+#[derive(Debug, Clone, Error)]
+#[error("{kind} at {span}")]
+pub struct ParseError {
+    pub kind: ParseErrorKind,
+    pub span: Span,
+}
+
+impl ParseError {
+    pub fn new(kind: ParseErrorKind, span: Span) -> Self {
+        Self { kind, span }
+    }
+}
+
+#[derive(Debug, Clone, Error)]
+pub enum ParseErrorKind {
+    #[error("unexpected token {got}, expected {expected}")]
+    UnexpectedToken { expected: String, got: String },
+
+    #[error("unexpected end of file")]
+    UnexpectedEof,
+
+    #[error("invalid expression starting with {0}")]
+    InvalidExprStart(String),
+
+    #[error("invalid type expression: {0}")]
+    InvalidTypeExpr(String),
+
+    #[error("invalid pattern: {0}")]
+    InvalidPattern(String),
+
+    #[error("expected identifier, got {0}")]
+    ExpectedIdent(String),
+}
+
+impl ParseError {
+    pub fn expected(expected: impl Into<String>, got: &Token, span: Span) -> Self {
+        Self::new(
+            ParseErrorKind::UnexpectedToken {
+                expected: expected.into(),
+                got: got.to_string(),
+            },
+            span,
+        )
+    }
+
+    pub fn eof(span: Span) -> Self {
+        Self::new(ParseErrorKind::UnexpectedEof, span)
+    }
+}