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.

.gitignore

@@ -1 +1,41 @@
+# Build artifacts
 /target
+**/target/
+*.dylib
+*.so
+*.dll
+*.wasm
+
+# Cargo
+Cargo.lock
+
+# Node
+node_modules/
+package-lock.json
+dist/
+.next/
+
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+*~
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Logs
+.logs/
+
+# Worktrees
+.worktrees/
+
+# Swift build
+.build/
+*.xcuserdata
+DerivedData/
+
+# WASM
+pkg/