re: built-in 12 is activate_unit — 517/517 on the ordering test

The highest-traffic unnamed built-in: 1197 sites across all 28 stages.
sub_822659F0 read directly:

* indexes [phase+324]'s record array by the slot-4 symbol;
* returns 0 immediately when the live object [record+4] is NULL, so it
  registers an object that already exists rather than spawning one;
* sets [record+16] = 2, the documented "active" state every unit predicate
  tests;
* stores sub_82301118's packed result into [record+20] (low 16) and
  [record+24] (high 16) -- member counts;
* posts opcode 513 (0xAB0201BA) either way; the slot-8 mode (1 in 999 sites,
  0 in 198) only decides whether cmd+20 is also set to 1.

The ordering test: if this activates a unit for the script, no predicate
should test a unit before it. Over all 28 stages, for every (stage, unit)
pair having both, activate_unit comes first 517 times and a predicate first
0 times. Recorded as file order rather than proven execution order --
coroutines can interleave -- but 517 with no exceptions is not a coincidence.
344 units are tested without ever being activated (live from mission start)
and 203 activated without being tested.

Also recorded: a tidy closure that FAILS. squad_survival_pct reports current
over initial and activate_unit snapshots counts, so the snapshot looks like
the baseline. It is not -- built-in 24 reads [record+16] for the state then
calls sub_823011B0 and sub_82301118 on the LIVE object, never touching
+20/+24. What reads those two fields is unidentified.

Named coverage is now 57 of 108 distinct ids and ~80% of call-site traffic.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PMRJjbxLqZtsb5Vb7KunPE

docs/re/structures/isl-builtins.md

@@ -140,6 +140,47 @@ and spread checks but its maximum *exceeds* the table — flag indices run 0..31
 against symbol tables as small as 40 — so it is excluded, and the disassembler
 now declines to resolve it rather than printing an invented name.
 
+### ✅ Built-in 12 is `activate_unit` — 1197 sites, all 28 stages
+
+The highest-traffic unnamed built-in. `sub_822659F0`, read directly:
+
+* indexes `[phase+324]`'s record array by the slot-4 symbol;
+* **returns 0 immediately when the live object `[record+4]` is NULL** — so it
+  registers an object that already exists; it does **not** spawn one;
+* sets **`[record+16] = 2`**, the documented *active* state that every unit
+  predicate tests;
+* stores `sub_82301118`'s packed result into `[record+20]` (low 16 bits) and
+  `[record+24]` (high 16) — member counts;
+* posts interpreter opcode **513** (`0xAB0201BA`) either way; the slot-8 mode
+  (1 in 999 sites, 0 in 198) only decides whether `cmd+20` is also set to 1.
+
+#### ✅ The ordering test — 517 / 517
+
+If this activates a unit for the script, no predicate should ever test a unit
+before it. Over all 28 stages, for every (stage, unit) pair having both an
+`activate_unit` call and a predicate (`unit_state`, `hp_pct_test`, `unit_alive`,
+`unit_hp_pct`, `dist_lt`, `unit_relation`) on that same unit:
+
+| | count |
+|---|---|
+| `activate_unit` comes first | **517** |
+| a predicate comes first | **0** |
+
+🟡 That is *file* order, not proven execution order — coroutines mean the two can
+in principle interleave. But 517 with zero exceptions across 28 stages is not a
+coincidence.
+
+344 units are tested without ever being activated (they are live from mission
+start) and 203 are activated without being tested.
+
+#### ❌ It is NOT the survival-percentage baseline
+
+The tempting closure: `squad_survival_pct` (id 24) reports current ÷ initial, and
+`activate_unit` snapshots counts, so the snapshot must be the baseline. **It is
+not.** Built-in 24 reads `[record+16]` for the state and then calls
+`sub_823011B0` and `sub_82301118` on the **live object** — it never touches
+`+20`/`+24`. ❔ What reads those two fields is unidentified.
+
 ### ✅ Built-in 15 is `set_group_speed` — the group's commanded speed
 
 `15` is the biggest unnamed built-in: **1360 sites across 27 of 28 stages**. Its