docs: REQUIREMENTS.md — OpenGL 4.3 floor, disk/RAM planning

Acer M5-481T fails Ghostty because Ivy Bridge HD 4000 tops out at
OpenGL 4.2 in Mesa; Ghostty needs 4.3 — a generation ceiling, not a
missing driver. Document that plus UEFI/x86_64, RAM, and storage floors
(Nix store generations + BTRFS snapper + hibernate swap≈RAM). Linked
from README, docs map, HARDWARE.md; #95 points here.

agent/BACKLOG.md

@@ -149,6 +149,13 @@ browser posture is explicit).
 **Machine:** Acer Aspire M5-481T only. **Dell XPS 9350 starts Ghostty fine**
 (Bernardo clarified 2026-07-15 — do not treat XPS as a Ghostty failure).
 
+**Why this hardware fails:** the Aspire is Ivy Bridge–class (**Intel HD
+4000**, ~2012). Mesa’s max for that GPU is **OpenGL 4.2**; Ghostty requires
+**4.3**. That is a **generation ceiling**, not a missing package — no
+Nomarchy or Mesa tweak can make HD 4000 report 4.3 honestly. Documented in
+`docs/REQUIREMENTS.md`. The product response is a fallback terminal (or
+probe), not “fix Intel drivers.”
+
 On the Acer, Bernardo installed **kitty** by hand (worked) and launched
 Ghostty from that shell twice — **same box, two lifecycle points, two log
 shapes**: