docs(v4.0.0): the word card stays as v3; where a v4 node can hook it

Captain Bob reversed the change the same day: leave the word card exactly
as v3, if a place to hook can be found.  Records that there is one -- the
call opcode, one place in the engine -- and what does not pass through it:
62 in-line words, EXECUTE, hand-written jumps, and call targets that are
not dictionary entries.  Notes that v3's TTL is computed from per-word
heat.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/v4.0.0/V3-PARITY.md

@@ -256,8 +256,44 @@ pretty much our division point is going to be. We're going to block things
 off and we're going to build little teeny teeny machines everywhere on the
 fabric."
 
-So the word card changes with the unit of heat. What it becomes is not yet
-settled; see the question put the same day.
+**Reversed the same day:** "Now that I think about it, let's leave it
+exactly as we're doing it in v3 — if we can figure out where to hook into
+it to measure that."
+
+So the word card stays per word, as v3, provided a v4 node has a place to
+check it.
+
+**Where a v4 node can hook, from the code:**
+
+- There is one. Every call of one word by another is the `call` opcode,
+  executed at one place in the engine: `v4/src/exec.c`, `case V4_OP_CALL`.
+  The node has the target's address there, before control moves. It is the
+  line where the per-call-target count was taken.
+- The compiler lays down a `call` for every word that is not in-line
+  (`(CALL,)`, `v4/capsule/compile.v4:100`) and never turns a call into a
+  jump, so definitions compiled on the node all pass through it.
+
+**What does not pass through it:**
+
+- **In-line words.** 62 words are compiled as opcodes, not called, among
+  them `DUP DROP OVER SWAP ROT + - AND XOR OR @ ! +! 1+ 1- 2+ 2- NEGATE`.
+  v3 checks every one of these at every execution. Six must stay in line
+  whatever is decided, because a call would bury the return address:
+  `>R R> R@ I J LEAVE`.
+- **`EXECUTE`.** It is `push ;` — the word is entered by a return, not a
+  call.
+- **Hand-written jumps.** The assembled nucleus ends many words by jumping
+  to another (`: CR 10 jump EMIT`). A colon definition in a capsule does
+  not, so this shrinks as words move to the capsule.
+- **Telling a word from a mere address.** Not every call target is a
+  dictionary entry: the assembled files define more routines than they
+  give headers to. The node must be able to tell, at the call, whether the
+  target has the three cells of an entry before it.
+
+**And what v3 measures.** v3 works a word's TTL out from that word's heat
+(`heat/4 + 256`). Section 2.7 rules heat per opcode. A word card exactly as
+v3's needs a count per word, which the same hook could take; whether it
+does is not settled.
 
 ## 2. Row 3: physics and per-word heat