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>

.ghci

@@ -1,9 +1,8 @@
 :set -XNoImplicitPrelude
 :def loadFromIHP \file -> (System.Environment.getEnv "IHP_LIB") >>= (\ihpLib -> readFile (ihpLib <> "/" <> file))
 :loadFromIHP applicationGhciConfig
--- Resource limit: override IHP's default -j (unlimited parallel) with -j1
--- on this constrained host (2 CPU / 3.8 GiB RAM).
-:set -j1
+-- haskelseed (8 CPU / 30 GiB RAM): allow parallel module compilation.
+:set -j4
 -- Report errors from all modules in one pass (don't stop at first failure).
 -- Critical for batch error fixing — see IHUB-WP-0016 C4.
 :set -fkeep-going