Add app-lib library package to reduce nix build times

[mpscholten]

Summary

• Introduces an intermediate appLibPackage that compiles all app modules once as a Cabal library, so entry points (RunProdServer, RunJobs, scripts) only compile 1-2 wrapper modules instead of the entire app
• For textcontent (157 modules, 2 scripts): 165 vs 628 module compilations, ~74% reduction
• Dev shell and ghci workflow unchanged

How it works

Before: Each of RunProdServer, RunJobs, and every script independently compiled all ~157 app modules via ghc --make:

modelsPackage -> runServer   (157 modules)
              -> runJobs     (158 modules)  
              -> script1     (157 modules)
              -> script2     (157 modules)

After: App modules are compiled once into a library. Entry points link against it:

modelsPackage -> appLibPackage (162 modules, once)
                    -> binaries: Main.hs (1 module) -> RunProdServer
                                 RunJobs.hs (1 module) -> RunJobs
                                 script wrappers (1 module each)

Key implementation details:

• appLibSrc generates a .cabal file at build time, discovering modules via find and populating build-depends from ghc-pkg list (filtering out private z-* internal sub-libraries)
• Config/ modules handled via hs-source-dirs: . Config (matching existing -i. -iConfig from Makefile)
• Test/ directory and Main.hs excluded from the library
• Entry point build deletes all .hs except Main.hs so GHC resolves imports from the library package DB instead of recompiling from source
• All entry points share -odir/-hidir for object file reuse (e.g. RunJobs reuses Main.o)

Test plan

• nix build produces RunProdServer, RunJobs, and script binaries
• Library build log shows all ~162 modules compiled
• Entry point build log shows only 1-2 modules compiled per binary
• Test on a second IHP project to verify generality

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