re: _EXn is the n-th challenge stage; the Extra0n join is refuted

Reading the docs first paid: unit-substructure-records already had
UN_e001_ADAN_Elan_EX4 and arsenal-item-weapon-chain already had _T_EX5_el, so
last pass's "the only two _EX5 names on the disc" was too narrow. Withdrawn and
replaced by a census.

Every name ENDING in _EX<digit> across all 41 archives, taken from parsed IDXD
record names, field names and string values (never a raw byte scan): 26 names,
two digits only - _EX4 (7 units, 6 ADAN + 1 TCAF) and _EX5 (4 TCAF units + 1
weapon), each with a UnitName_/WeaponCannonName_ twin.

The stage join is exact. _EX4 appears only in tables S27 declares
(EnumUnit_S27.tbl, UnitGroup_S27.tbl); _EX5 only in S28's (EnumUnit_S28.tbl,
UnitGroup_S28.tbl, EnumWeapon_EX5.tbl); the other 20 stages have neither. The
challenge missions are S24-S29 in order, so S27 is challenge #4 and S28 is
challenge #5: _EXn names the n-th challenge ("EXtra") mission's bespoke
variants. S28 declares both schemes at once - EnumUnit_S28.tbl by stage number,
EnumWeapon_EX5.tbl by challenge index, for the same mission.

Refuted: _EXn is not the Extra0n numbering. There are four Extra slots but _EX5
exists, and under that reading Extra04 would be S29, which has no _EX assets.

Makes available but does not settle result-screens' open "what does EX_ mean":
"the challenge-mission debriefing" fits its evidence that the EX_ twin drops
overview_rank/overview_medals. Recorded there as a reading, not adopted.

All fifteen artefacts byte-identical.

docs/re/BACKLOG.md

@@ -6186,3 +6186,41 @@ S28's own weapon table is a single `_EX5` AA-gun variant, and the only other
 
 🟡 Not settled: what `EX` stands for; the two unnamed `_Test` variant objects;
 what `Placement_*`/`RouteTest_*` are for.
+
+---
+
+## ✅❌ 2026-08-28 — `_EXn` is the n-th challenge stage; the `Extra0n` join is refuted
+
+Item (a) continued. Reading the docs first paid: `unit-substructure-records`
+already had `UN_e001_ADAN_Elan_EX4` and `arsenal-item-weapon-chain` already had
+`_T_EX5_el`, so last pass's "the only two `_EX5` names on the disc" was too
+narrow — withdrawn and replaced by a census.
+
+✅ Every name **ending in** `_EX<digit>` across all 41 archives, taken from
+parsed record names / field names / string values: **26 names, two digits only** —
+`_EX4` (7 units, 6 ADAN + 1 TCAF) and `_EX5` (4 TCAF units + 1 weapon), each
+with a `UnitName_`/`WeaponCannonName_` twin.
+
+✅ **The stage join is exact.** `_EX4` appears only in tables **S27** declares
+(`EnumUnit_S27.tbl`, `UnitGroup_S27.tbl`); `_EX5` only in **S28**'s
+(`EnumUnit_S28.tbl`, `UnitGroup_S28.tbl`, `EnumWeapon_EX5.tbl`); **the other 20
+stages have neither.** The challenge missions are S24–S29 in order, so S27 is
+challenge **#4** and S28 challenge **#5**: `_EXn` = the n-th challenge
+("EXtra") mission's bespoke variants. S28 declares both schemes at once —
+`EnumUnit_S28.tbl` by stage number, `EnumWeapon_EX5.tbl` by challenge index.
+
+❌ **Refuted: `_EXn` is not the `Extra0n` numbering.** There are four `Extra`
+slots but `_EX5` exists, and under that reading `Extra04` = S29, which has no
+`_EX` assets. (The six leaderboard modes do map onto the six challenge stages;
+`_EXn` counts stages.)
+
+🟡 Makes available, not adopted: `result-screens`' open "what does `EX_` mean" —
+"the challenge-mission debriefing" now fits its own evidence that the `EX_` twin
+drops `overview_rank`/`overview_medals`. Consistency, not proof; the selection
+has not been shown.
+
+🟡 Not settled: the four weapons declared by no weapon table — three of them
+(`_NoseGun_None`, `_NoseGun_Ttrl`, `Weapon_ADAN_Attacker_S_GunTurret`) are
+already in `unit-substructure-records`' "26 weapons no turret mounts", so they
+are unmounted **and** undeclared; `_Laser_Ttrl` is not on that list and is the
+one still to place.