re: two trigger appenders -- built-in 19's node carries the handler routine

Resolves the puzzle the previous commit left open: payload+28 is a 1-12 index, yet
the drain hands it to a spawner that wants a code offset.  The cause is that built-in
25 is not the only thing that appends a trigger node.

Searching for functions that write a double at +16 AND a word at +28 of the same
register finds exactly two: sub_8226A0D8 (built-in 25, vtable slot 28) which fills the
payload inline, and sub_8226E160, a free-list recycling variant whose sole caller
sub_82269ED0 is vtable slot 68 = BUILT-IN 19.

Built-in 19 passes local[4] -> node+0 (the unit), local[12] -> +4, local[24] -> +16 as
a double, and local[32] -> +28.  That last one is the field the drain spawns from, and
it is a genuine code offset:

  over all 79 call sites disc-wide
    land on the instruction stream    79 / 79 = 100.0%
    control, random 4-aligned offsets           27.8%
    values 12164 .. 111080, 73 distinct

Large, distinct, and every one resolves against a control that resolves barely a
quarter of the time.  So built-in 19 registers a trigger whose handler is a routine,
and the drain's `base + payload+28` spawn is that handler.  The two builders simply
put different things in the same slot.

Observed alongside: built-in 25 writes +24 = 1 and built-in 19 writes +24 = 0, which
is kind-tag shaped -- but the drain was not shown branching on it, so that is recorded
as an observation and not a reading.

All artefacts regenerate byte-identical; documentation only.

Still open: whether +24 selects between the node kinds, and this STILL does not
explain the unreached code -- 0 of the 79 handler offsets are unreached run-starts.

docs/re/structures/isl-trigger-node.md

@@ -85,13 +85,59 @@ argument. `sub_8226D740` does operate on the trigger container; what
 `sub_8226E458` does *to* it is still unread, so only the argument is corrected
 here, not the conclusion.
 
+## ✅ RESOLVED: there are TWO appenders, and only one carries a routine
+
+The puzzle above — `payload+28` holding a 1–12 index while the drain hands it to
+a spawner that wants a code offset — had a simple cause: **built-in 25 is not the
+only thing that appends a trigger node.**
+
+Searching for functions that write a **double at +16** and a **word at +28** of
+the same register finds exactly two:
+
+| builder | how |
+|---|---|
+| `sub_8226A0D8` — **built-in 25**, vtable slot 28 | fills the payload inline |
+| `sub_8226E160` | a free-list recycling variant, called from `sub_82269ED0` |
+
+`sub_82269ED0` is **vtable slot 68 = built-in 19**, and it calls the builder as
+
+```
+8226A034  addi r3, r28, 272      ; the trigger container
+8226A040  lwz  r4, 4(r30)        ; local[4]   -> node+0   (the unit)
+8226A03C  lwz  r5, 12(r30)       ; local[12]  -> node+4
+8226A038  lfd  f1, 24(r30)       ; local[24]  -> node+16  (double)
+8226A030  lwz  r8, 32(r30)       ; local[32]  -> node+28
+```
+
+### ✅ Built-in 19's `local[32]` IS a code offset
+
+| over all 79 call sites disc-wide | |
+|---|---|
+| land on the instruction stream | **79 / 79 = 100.0 %** |
+| control, random 4-aligned offsets | 27.8 % |
+| value range | 12 164 … 111 080, **73 distinct** |
+
+Large, distinct, and every one resolves — against a control that resolves barely
+a quarter of the time. So **built-in 19 registers a trigger whose handler is a
+routine**, and the drain's `base + payload+28` spawn is exactly that handler.
+
+Built-in 25's `payload+28` really is a small index; the two builders simply put
+different things in the same slot. Observed alongside it: built-in 25 writes
+**`+24 = 1`** and built-in 19 writes **`+24 = 0`**, which is the shape of a kind
+tag — 🟡 but the drain was not shown branching on it, so that is an observation,
+not a reading.
+
 ## 🟡 Not settled
 
 * **What `local[12]` indexes.** Values 1–12, and `SYM1_SLOTS` lists slot 12 for
   built-in 25 (symbol table 1 — routes, messages, objectives), which is
   consistent with a small index but was not verified here.
-* **What the drain actually spawns.** `[stack+112]` is filled from `payload+28`,
-  yet the spawner takes a code offset — so some step in that chain is not what I
-  read. Left open rather than guessed.
+* ~~What the drain actually spawns.~~ ✅ Resolved above: built-in 19's nodes carry
+  a real routine offset at `+28`.
+* **Whether `+24` selects between the two node kinds** — 1 from built-in 25, 0
+  from built-in 19 — is unshown; the drain reads it into a stack slot that the
+  spawn path does not visibly use.
+* **This still does not explain the unreached code.** All 79 of built-in 19's
+  handler offsets are already reachable — **0 are unreached run-starts**.
 * **`sub_8226E458`'s effect** on the container.
 * **What starts the ~15 % of unreached code** — still open, and this was not it.