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
 ```