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/src/pipeline.rs

@@ -0,0 +1,40 @@
+use crate::context::EvalContext;
+use crate::interpreter::evaluate;
+use crate::lexer::tokenize;
+use crate::parser::parse;
+use crate::span::Span;
+use crate::types::CalcResult;
+
+/// Evaluate a single line of input and return the result.
+pub fn eval_line(input: &str, ctx: &mut EvalContext) -> CalcResult {
+    let trimmed = input.trim();
+    if trimmed.is_empty() {
+        return CalcResult::error("empty input", Span::new(0, 0));
+    }
+
+    let tokens = tokenize(trimmed);
+
+    // Check for text-only or comment-only lines
+    let has_expr = tokens.iter().any(|t| {
+        !matches!(
+            t.kind,
+            crate::token::TokenKind::Text(_)
+                | crate::token::TokenKind::Comment(_)
+                | crate::token::TokenKind::Eof
+        )
+    });
+    if !has_expr {
+        return CalcResult::error("no expression found", Span::new(0, trimmed.len()));
+    }
+
+    match parse(tokens) {
+        Ok(expr) => evaluate(&expr, ctx),
+        Err(e) => CalcResult::error(&e.message, e.span),
+    }
+}
+
+/// Evaluate multiple lines of input, sharing context across lines.
+/// Variable assignments on earlier lines are visible to later lines.
+pub fn eval_sheet(lines: &[&str], ctx: &mut EvalContext) -> Vec<CalcResult> {
+    lines.iter().map(|line| eval_line(line, ctx)).collect()
+}