Document the fence a calc operand gets, and trim two comments

The one-spelling section described a calc operand only as a route that
renders a level, so a reader could not predict that {..cat.net} beside
{calc(net * 2, 2)} is now a load error with no path token in sight. It says
so, and says what an operand's draw covers and what it leaves free.

Dropped the paragraph in checkBoundLevelsHeld that restated its own first
line and the loop below it, and the escape-analysis note in expand.

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

README.md

@@ -363,6 +363,12 @@ always agrees, and the other is a load error. This covers every way a level can
 be rendered: a token, a `{calc()}` operand, and a `{..path}` reference — wherever
 the reference sits, including in a field the format renders.
 
+A `{calc()}` operand is held on its own terms too, so the same fence guards it:
+`{..cat.net} x 2 = {calc(net * 2, 2)}` names one field two ways and is a load
+error, with no path token anywhere. What an operand's draw fixes is the value it
+renders, so a reference reaching *through* it is caught as well, while a sibling
+its format never renders is free — that sibling is no part of the value.
+
 ```text
 token {p} renders a level that {p.first} reads a path into; name the fields you want instead
 ```

reference.go

@@ -44,9 +44,6 @@ func linkRefs(root map[string]node) error {
 // format (a token, a calc operand); this settles the rest — a reference, whether it
 // sits in that format or in anything the format renders, however deep.
 //
-// It walks t.held, so it covers both kinds of hold: the level a dotted token reads,
-// and the sibling a {calc()} reads.
-//
 // It runs after checkNoCycles, whose guarantee is what lets the walk terminate.
 func checkBoundLevelsHeld(root map[string]node) error {
 	return walkNodes(root, func(path string, n node) error {

render.go

@@ -138,8 +138,7 @@ func expand(s *session, t *template) string {
 	var b strings.Builder
 	b.Grow(t.grow)
 	// One draw per held name, for this expansion only: a nested template and each
-	// repeat iteration get their own, since each is its own expansion. held reaches
-	// no further than readField, which keeps it and its maps on the stack.
+	// repeat iteration get their own, since each is its own expansion.
 	var held *draws
 	if len(t.held) > 0 {
 		held = &draws{