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-manifest/src/error.rs

@@ -0,0 +1,39 @@
+//! Manifest error types.
+
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum ManifestError {
+    #[error("io error: {0}")]
+    Io(#[from] std::io::Error),
+
+    #[error("toml parse error: {0}")]
+    Toml(#[from] toml::de::Error),
+
+    #[error("semver parse error for '{field}': {source}")]
+    Semver {
+        field: String,
+        #[source]
+        source: semver::Error,
+    },
+
+    #[error("missing required field: {0}")]
+    MissingField(String),
+
+    #[error("invalid value for '{field}': {reason}")]
+    InvalidValue { field: String, reason: String },
+
+    #[error("el.toml not found (searched from {0})")]
+    NotFound(String),
+
+    #[error("invalid cross target '{0}': use x86_64-linux, aarch64-linux, x86_64-macos, aarch64-macos, wasm32")]
+    InvalidCrossTarget(String),
+
+    #[error("invalid build target '{0}': use debug, release, prod")]
+    InvalidBuildTarget(String),
+
+    #[error("invalid seal key source '{0}': use env:VAR, file:path, or literal")]
+    InvalidSealKeySource(String),
+}
+
+pub type ManifestResult<T> = Result<T, ManifestError>;