re: the 59-of-131 arsenal question is closed -- an item names a hardpoint

An Arsenal item does not reference a Weapon record.  It references a
Turret_NNN HARDPOINT SLOT on the player craft's own unit table, and the
slot is what carries the WeaponID.  Three hops:

  Arbalest_155KG.PlayerWeapon -> Turret_050  (a slot on
  UN_f001_TCAF_DeltaSaber_T_Player) -> .WeaponID ->
  Weapon_DSaber_P_wep_50_Cannon

Controls, both in the same loop: 0/59 distinct PlayerWeapon values are a
Weapon.ID; 59/59 are a Turret_NNN slot id; the full chain lands on a
Weapon.ID 59/59.  WingmanWeapon resolves identically.  The WEAPONS
roster's 59 = 55 item names + 4 empty-slot sentinels.

Wingmen fly a cheaper gun: following the same 59 slots across craft
variants, the _Player tables give each item its own weapon record (59
distinct) while the AI tables collapse all 59 onto 10 generic classes.
That is most of the 131.

Upgrades yesterday's 'hardpoint catalogue' reading from 21 to adopted,
proved from an independent file, and corrects its '10 distinct WeaponID'
figure -- that was the AI variant, not the player's.

Also adds an __main__ guard to unit_substructures.py so importing
pak_entries from it does not run its report; its artefact is unchanged
and still byte-identical.

docs/re/data/arsenal-chain.txt

@@ -0,0 +1,72 @@
+# The Arsenal item -> hardpoint slot -> Weapon record chain
+# Regenerate: python3 tools/re-capture/arsenal_chain.py
+# See docs/re/structures/arsenal-item-weapon-chain.md
+
+## the roster and the item records
+   WEAPONS roster values       59
+   arsenal ITEM records        59
+   item names that are roster values   55
+   roster values with no item record: ['No_Equipment', 'NullWeapon_Arm1', 'NullWeapon_Arm2', 'NullWeapon_Arm3']
+   Weapon.ID values in the datasheet  131
+
+## CONTROL  item.PlayerWeapon is NOT a Weapon.ID
+   distinct PlayerWeapon values  59 ; how many are a Weapon.ID: 0
+
+## CONTROL  item.PlayerWeapon IS a Turret_NNN slot id
+   UN_f001_TCAF_DeltaSaber_T                slots 63  Player 59/59  Wingman 59/59
+   UN_f001_TCAF_DeltaSaber_T_EX5            slots 63  Player 59/59  Wingman 59/59
+   UN_f001_TCAF_DeltaSaber_T_EX5_el         slots 63  Player 59/59  Wingman 59/59
+   UN_f001_TCAF_DeltaSaber_T_Player         slots 63  Player 59/59  Wingman 59/59
+   UN_f001_TCAF_DeltaSaber_T_Player_Ttrl1   slots  5  Player  3/59  Wingman  3/59
+   UN_f001_TCAF_DeltaSaber_T_Player_Ttrl2   slots  5  Player  1/59  Wingman  1/59
+   UN_f001_TCAF_DeltaSaber_T_Ttrl           slots  4  Player  4/59  Wingman  4/59
+   UN_f002_TCAF_DeltaSaber_W                slots 63  Player 59/59  Wingman 59/59
+   UN_f002_TCAF_DeltaSaber_W_Player         slots 63  Player 59/59  Wingman 59/59
+   UN_f004_TCAF_DeltaSaber_A_Player         slots 63  Player 59/59  Wingman 59/59
+
+## THE CHAIN, per craft variant: item -> slot -> WeaponID -> Weapon.ID
+   UN_f001_TCAF_DeltaSaber_T                 59/ 59 land on a Weapon.ID, 10 distinct
+        slot WeaponIDs: 59 non-NULL, 10 distinct, e.g. ['Weapon_TCAF_DeltaSaber_ASMissile', 'Weapon_TCAF_DeltaSaber_Beam']
+   UN_f001_TCAF_DeltaSaber_T_EX5             59/ 59 land on a Weapon.ID, 10 distinct
+        slot WeaponIDs: 59 non-NULL, 10 distinct, e.g. ['Weapon_TCAF_DeltaSaber_ASMissile', 'Weapon_TCAF_DeltaSaber_Beam']
+   UN_f001_TCAF_DeltaSaber_T_EX5_el          59/ 59 land on a Weapon.ID, 10 distinct
+        slot WeaponIDs: 59 non-NULL, 10 distinct, e.g. ['Weapon_TCAF_DeltaSaber_ASMissile', 'Weapon_TCAF_DeltaSaber_Beam']
+   UN_f001_TCAF_DeltaSaber_T_Player          59/ 59 land on a Weapon.ID, 59 distinct
+        slot WeaponIDs: 59 non-NULL, 59 distinct, e.g. ['Weapon_DSaber_P_wep_01_Beam', 'Weapon_DSaber_P_wep_02_Missile']
+   UN_f001_TCAF_DeltaSaber_T_Player_Ttrl1     3/ 59 land on a Weapon.ID,  3 distinct
+        slot WeaponIDs: 3 non-NULL, 3 distinct, e.g. ['Weapon_DSaber_P_wep_01_Beam', 'Weapon_DSaber_P_wep_02_Missile']
+   UN_f001_TCAF_DeltaSaber_T_Player_Ttrl2     1/ 59 land on a Weapon.ID,  1 distinct
+        slot WeaponIDs: 1 non-NULL, 1 distinct, e.g. ['Weapon_DSaber_P_wep_01_Beam']
+   UN_f001_TCAF_DeltaSaber_T_Ttrl             4/ 59 land on a Weapon.ID,  4 distinct
+        slot WeaponIDs: 4 non-NULL, 4 distinct, e.g. ['Weapon_TCAF_DeltaSaber_ASMissile', 'Weapon_TCAF_DeltaSaber_Beam']
+   UN_f002_TCAF_DeltaSaber_W                 59/ 59 land on a Weapon.ID, 10 distinct
+        slot WeaponIDs: 59 non-NULL, 10 distinct, e.g. ['Weapon_TCAF_DeltaSaber_ASMissile', 'Weapon_TCAF_DeltaSaber_Beam']
+   UN_f002_TCAF_DeltaSaber_W_Player          59/ 59 land on a Weapon.ID, 59 distinct
+        slot WeaponIDs: 59 non-NULL, 59 distinct, e.g. ['Weapon_DSaber_P_wep_01_Beam', 'Weapon_DSaber_P_wep_02_Missile']
+   UN_f004_TCAF_DeltaSaber_A_Player          59/ 59 land on a Weapon.ID, 59 distinct
+        slot WeaponIDs: 59 non-NULL, 59 distinct, e.g. ['Weapon_DSaber_P_wep_01_Beam', 'Weapon_DSaber_P_wep_02_Missile']
+
+   player craft slots not referenced by any item: ['Turret_000', 'Turret_097', 'Turret_098', 'Turret_099']
+
+## a full item record -- Arbalest_155KG
+     Dependency             
+     MissionObjective       
+     Model                  rou_f001_wep_50_hangar
+     Package                rou_f001_wep_50_hangar.xpr
+     PlayerLOD              Equip_EnumLODSet_wep_50.tbl
+     PlayerMotion           Equip_EnumGameModel_wep_50.tbl
+     PlayerPackage          rou_f001_wep_50.xpr
+     PlayerWeapon           Turret_050
+     Points                 25000
+     Power                  B
+     Range                  B
+     SilhouetteModel        rou_f001_wep_50_NA
+     Size                   L
+     Speed                  -
+     Stage                  6
+     Type                   g04b
+     Weight                 1.5
+     WingmanLOD             Equip_EnumLODSet_wep_50.tbl
+     WingmanMotion          Equip_EnumGameModel_wep_50.tbl
+     WingmanPackage         rou_f001_wep_50.xpr
+     WingmanWeapon          Turret_050