Hold what a calc operand's render settled, up to a reference

The fence pinned only the operand node, so a reference reaching into it drew
that value again: {net.v} was a load error while {..cat.net.v} loaded and
disagreed in 20 of 40 renders. The hold now covers what rendering the operand
settles inside itself, so both spellings of that shape are rejected.

The walk stops at a {..path} edge, where the operand's own value ends and a
shared source begins. cover stops there too, through named, so both halves of
the fence end at the same boundary — and two names drawing from one referenced
category stay two draws.

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

README.md

@@ -365,10 +365,12 @@ 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. A calc renders its operand whole, so what the
-hold pins is that one field — another route conflicts only by naming it. Two
-names that merely draw from one source are two draws, as `{word} {word}` is, so
-two dice over one `{..die}` are fine.
+error, with no path token anywhere. A calc renders its operand whole, so the hold
+pins every value that render settled: naming the operand and reaching into it
+both conflict, and `{net.v}` and `{..cat.net.v}` are rejected alike. The hold
+ends at a `{..path}`, where the operand's own value ends and a shared source
+begins — two names drawing from one referenced category are two draws, as
+`{word} {word}` is, so two dice over one `{..die}` are fine.
 
 ```text
 token {p} renders a level that {p.first} reads a path into; name the fields you want instead