fix(build): route TH to ghc-iserv-dyn to bypass truncated libHSghc.a

Root cause: libHSghc-9.10.3-5702.a (287,768,576 bytes) has its last AR
entry (Expr.o) claiming 517,544 bytes but only 82,258 bytes remain —
the archive is truncated. GHC's internal readAr (Data.Binary.Get) panics
at position 287,686,318 when it tries to read the full claimed size.

The truncated .a is read lazily: IHP's TH splices queue a dependency on
the ghc package, which flushes to readAr after all 477 modules compile.
This explains the invariant crash at [477 of 477] WidgetVersionInclude.

ghc-iserv-dyn is not exposed in ghc-with-packages/bin/ (why
-fexternal-interpreter alone silently fell back to the internal linker).
Use -pgmi with the absolute path in the unwrapped GHC store to force
iserv-dyn, which uses dlopen on libHSghc.so (intact, 110 MB) instead
of readAr on the truncated .a. No crash.

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

flake.nix

@@ -127,6 +127,18 @@
                                                "--disable-split-sections"
                                                "--ghc-option=-j1"
                                                "--disable-shared"
+                                               # GHC 9.10.3 bug: libHSghc-9.10.3-5702.a is truncated (last AR
+                                               # entry Expr.o claims 517544 bytes but only 82258 remain).
+                                               # GHC's internal static linker (readAr via Data.Binary.Get) panics
+                                               # after all 477 modules compile when it flushes deferred symbol
+                                               # loads from IHP's TH splices that transitively need the ghc pkg.
+                                               # Fix: delegate TH evaluation to ghc-iserv-dyn, which uses dlopen
+                                               # on libHSghc.so (intact) instead of readAr on the truncated .a.
+                                               # ghc-iserv-dyn is not in ghc-with-packages/bin/, so use -pgmi
+                                               # with the absolute path in the unwrapped GHC store path.
+                                               "--ghc-option=-fexternal-interpreter"
+                                               "--ghc-option=-pgmi"
+                                               "--ghc-option=${hprev.ghc}/lib/ghc-9.10.3/bin/ghc-iserv-dyn"
                                            ];
                                            postUnpack = (old.postUnpack or "") + ''
                                                _actual="$sourceRoot/build/Generated/ActualTypes.hs"