feat(build): tune GHC for haskelseed (8 CPU / 30 GiB)

- .ghci: -j1 → -j4 for parallel module compilation
- scripts/compile-check: auto-detect high-RAM hosts (>16GB) and
  set GHCRTS="-A256m -M20g" to reduce GC pressure
- devenv.nix: update comment only (GHCRTS still set at runtime)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/compile-check

@@ -2,7 +2,7 @@
 # scripts/compile-check — one-shot GHC compilation check.
 #
 # Does NOT start postgres or tailwind — just GHC.
-# Relies on .ghci (sets -j1, -fkeep-going, loads IHP config).
+# Relies on .ghci (sets -j4, -fkeep-going, loads IHP config).
 #
 # Usage:
 #   scripts/compile-check            # show errors in terminal + write log
@@ -30,6 +30,14 @@ echo "[compile-check] Log: $LOGFILE"
 echo "[compile-check] Mode: ${MODE:-interactive}"
 echo "[compile-check] IHP_LIB: $IHP_LIB"
 
+# Override GHC RTS for hosts with more RAM (e.g., haskelseed: 8 CPU / 30 GiB).
+# devenv.nix sets -M2g for constrained hosts; override here if available RAM is higher.
+TOTAL_RAM_MB=$(awk '/MemTotal/{print int($2/1024)}' /proc/meminfo 2>/dev/null || echo 0)
+if [[ "$TOTAL_RAM_MB" -gt 16000 ]]; then
+    export GHCRTS="-A256m -M20g"
+    echo "[compile-check] Host RAM: ${TOTAL_RAM_MB}MB — using GHCRTS=$GHCRTS"
+fi
+
 # One-shot compile: load all modules via .ghci config then quit immediately.
 # -fkeep-going (set in .ghci) ensures all errors are reported, not just the first.
 # ghcid is not available in this environment; ghci one-shot gives the same error output.