docs: reconcile the 'free items are granted' wording with the 0 P purchase the probe caught

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/re/arsenal-develop-economy.md

@@ -77,10 +77,10 @@ economy:
 - gates are **stage completion** (`Complete Stage 1 … 15`), **a predecessor
   item** (`Completed development of the …`), or **an ace kill** (`Shoot down the
   ace pilot in Stage 11/13/14`);
-- costs run 3 000 → 350 000 points, and **twenty items have no cost at all** —
-  they are granted the moment their gate opens (`Tomahawk Alpha Rail Gun —
-  Complete Stage 1`), which is why they read as `Developed` in a list the player
-  never spent points in;
+- costs run 3 000 → 350 000 points, and **twenty items have no cost at all**
+  (`Tomahawk Alpha Rail Gun — Complete Stage 1`). Those are still *bought*, for
+  **0 P** — see the probe below, which caught one at `0 P` — which is why they
+  read as `Developed` in a list the player never spent points in;
 - two items are anomalies worth flagging: `Adhesive Mine B2A` says "Must spend
   points to develop" with **no number**, and ten late entries (`Ballista GSH`,
   `Designator LH`, `Smoke Marker Launcher`, `Bumble UV`, `Ballista 24 Rocket