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.

calcpad-engine/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "calcpad-engine"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+crate-type = ["cdylib", "staticlib", "rlib"]
+
+[dependencies]
+chrono = "0.4"
+dashu = "0.4"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"