feat(engine): establish calcpad-engine workspace with Epic 1 modules

Cherry-picked and integrated the best code from 105 parallel epic
branches into a clean workspace structure:

- calcpad-engine/: Core Rust crate with lexer, parser, AST,
  interpreter, types, FFI (C ABI), pipeline, error handling,
  span tracking, eval context (from epic/1-5 base)
- calcpad-engine/src/number.rs: Arbitrary precision arithmetic
  via dashu crate, WASM-compatible, exact decimals (from epic/1-4)
- calcpad-engine/src/sheet_context.rs: SheetContext with dependency
  graph, dirty tracking, circular detection, cheap clone (from epic/1-8)
- calcpad-wasm/: Thin WASM wrapper crate via wasm-bindgen,
  delegates to calcpad-engine (from epic/1-6)
- Updated .gitignore for target/, node_modules/, build artifacts
- Fixed run-pipeline.sh for macOS compat and CalcPad phases

79 tests passing across workspace.

Cargo.toml

@@ -1,7 +1,6 @@
-[package]
-name = "calcpad-engine"
-version = "0.1.0"
-edition = "2021"
-description = "Core calculation engine for CalcPad — a modern notepad calculator"
-
-[dependencies]
+[workspace]
+members = [
+    "calcpad-engine",
+    "calcpad-wasm",
+]
+resolver = "2"