Real control flow for "Mit Schleife" functions: DO/END, IF/ELSE/END, RETURN anywhere

Replaces the previous FOR-only, single-statement-body, static-unrolling
design with genuine control flow: FOR var IN source DO <block> END loop
bodies with any number of statements (including nested loops/IFs),
IF cond THEN <block> [ELSE <block>] END branching (nestable in any
block), and RETURN as an ordinary statement usable anywhere -- it
aborts execution immediately with its value, including breaking out of
a loop early.

This required a real, if small, architecture change: an IF branch or an
early RETURN depends on actual cell values, which the previous "unroll
everything into one static formula at parse time" approach can't
resolve (loop *counts* are knowable up front from a range's size;
branch *outcomes* aren't). A kind: 'block' call is now inlined only as
far as resolving nested function calls (expandBlock, parse-time, same
inside-out principle as before) and wrapped in a new BlockCallNode;
its own control flow is interpreted at evaluation time by a small
tree-walking interpreter added to evaluator.ts (execBlock/execStatement,
a thrown ReturnSignal for early exit), reusing userFunctions.ts's
substitute() (now exported) to resolve parameter/local-variable names
against the current environment. Dependency tracking stays fully
correct and conservative: collectRefs() walks into a BlockCallNode's
block (new collectRefsInBlock), so a cell referenced only in a branch
that doesn't happen to run is still a tracked precedent -- same
principle as an ordinary IF() in any spreadsheet.

Also added: comparison operators (= <> > < >= <=, result 1/0) as a new,
loosest-binding precedence level in the *general* expression grammar
(not just inside a block's IF) -- needed for IF conditions to be useful
at all. A definition-time static check (blockAlwaysReturns) rejects a
function at save time when it's not obviously guaranteed to reach
RETURN on every path (an IF without an ELSE, or one where only one
branch returns); the interpreter's own runtime "ended without RETURN"
-> #ERR remains the actual enforcement for cases the static check can't
see (e.g. a loop over an empty range).

Nested loops (FOR inside FOR) and nested IFs work as an emergent
property of the recursive statement grammar -- verified with dedicated
tests, not just assumed.

257 tests passing (30 net new/rewritten for this change), vue-tsc/
eslint/build clean. Verified live in the running app: a FIRSTOVER(range;
threshold) function using a FOR loop with a nested IF and an early
RETURN, confirmed it returns the first match, confirmed changing a
cell *past* the match point doesn't affect the result while changing
the match point itself does, and confirmed a function without a
guaranteed RETURN is rejected inline at save time.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/engine/index.ts

@@ -40,6 +40,7 @@ export {
   collectCalledFunctionNamesInBlock,
   declaredBlockNames,
   findInvalidForSource,
+  blockAlwaysReturns,
   type FunctionDef,
 } from './userFunctions';
 export {