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/source_map.rs

@@ -0,0 +1,57 @@
+//! Source map: maps bytecode instruction indices to source spans.
+//!
+//! Only emitted for the debug target. The JSON format is simple and can be
+//! consumed by any debugger or IDE extension.
+
+use serde::{Deserialize, Serialize};
+use el_lexer::Span;
+
+/// A single mapping entry: bytecode index → source span.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct MapEntry {
+    /// Index of the bytecode instruction (0-based).
+    pub instruction: usize,
+    pub start: usize,
+    pub end: usize,
+    pub line: u32,
+    pub col: u32,
+}
+
+impl MapEntry {
+    pub fn new(instruction: usize, span: Span) -> Self {
+        Self {
+            instruction,
+            start: span.start,
+            end: span.end,
+            line: span.line,
+            col: span.col,
+        }
+    }
+}
+
+/// The full source map for a compilation unit.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct SourceMap {
+    pub entries: Vec<MapEntry>,
+}
+
+impl SourceMap {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Record that instruction at `index` was generated from `span`.
+    pub fn record(&mut self, index: usize, span: Span) {
+        self.entries.push(MapEntry::new(index, span));
+    }
+
+    /// Look up the source span for a given instruction index.
+    pub fn lookup(&self, index: usize) -> Option<&MapEntry> {
+        self.entries.iter().rfind(|e| e.instruction <= index)
+    }
+
+    /// Serialize to JSON string.
+    pub fn to_json(&self) -> Result<String, String> {
+        serde_json::to_string_pretty(self).map_err(|e| e.to_string())
+    }
+}