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.

run-pipeline.sh

@@ -336,7 +336,7 @@ phase_dev() {
   local dev_log="${LOG_DIR}/${TIMESTAMP}-dev-${story_name}.log"
   run_claude \
     "Developing: $story_name" \
-    "/bmad-dev-story $story_path" \
+    "You are running in a fully autonomous pipeline — do NOT ask questions, do NOT wait for input. If the story file is missing task breakdowns, create them yourself and proceed immediately. Always choose option 1 (do it yourself). /bmad-dev-story $story_path" \
     "$dev_log" \
     "$work_dir" || return 1
 
@@ -478,8 +478,26 @@ process_story_worktree() {
   fi
 
   log STEP "Creating worktree: $story_name → $branch"
-  git -C "$PROJECT_ROOT" branch "$branch" main 2>/dev/null || true
-  git -C "$PROJECT_ROOT" worktree add "$wt_path" "$branch" 2>/dev/null
+  # Reset branch to main if it already exists, or create fresh
+  if git -C "$PROJECT_ROOT" rev-parse --verify "$branch" &>/dev/null; then
+    git -C "$PROJECT_ROOT" branch -f "$branch" main 2>/dev/null || true
+    log INFO "Reset existing branch: $branch"
+  else
+    if ! git -C "$PROJECT_ROOT" branch "$branch" main 2>&1; then
+      log ERROR "[parallel] Failed to create branch: $branch"
+      echo "1" > "${RESULTS_DIR}/${story_name}.result"
+      return 1
+    fi
+  fi
+  # Remove stale worktree if path exists
+  if [ -d "$wt_path" ]; then
+    git -C "$PROJECT_ROOT" worktree remove --force "$wt_path" 2>/dev/null || rm -rf "$wt_path"
+  fi
+  if ! git -C "$PROJECT_ROOT" worktree add "$wt_path" "$branch" 2>&1; then
+    log ERROR "[parallel] Failed to create worktree: $wt_path"
+    echo "1" > "${RESULTS_DIR}/${story_name}.result"
+    return 1
+  fi
 
   # Symlink .claude/ so skills are available in worktree
   if [ -d "${PROJECT_ROOT}/.claude" ] && [ ! -e "${wt_path}/.claude" ]; then