Add ghc9123 variant and Windows TH crash stress harness

[zeme-wana]

What

The Windows (mingwW64) cross build of the plutus-core library intermittently fails with a GHC runtime crash while the Wine-hosted external interpreter (iserv) runs the cost-model Template Haskell splices ($$(readJSONFromFile ...) in PlutusCore.Evaluation.Machine.ExBudgetingDefaults):

iserv-proxy-interpreter.exe: internal error: scavenge_stack: weird activation record found on stack: 27
ghc-iserv terminated (1)

Example CI failure: https://ci.iog.io/build/13125016/nixlog/1

This PR adds a stress harness to measure how often the crash occurs per GHC toolchain:

• scripts/windows-th-crash-stress.sh — cycles cold rebuilds (nix build --rebuild, so the binary cache is never reused) of projectCross.mingwW64.hsPkgs.plutus-core.components.library across three arms — ghc96, ghc912, ghc9123 — round-robin, classifies each build (target crash / other iserv crash / other failure / success) and tallies results into a CSV + summary table. Crash logs are kept, success logs dropped. Run on Linux with the IOG binary cache.
• nix/project.nix — a ghc9123 flake variant (compiler-nix-name = "ghc9123").
• nix/shell.nix / nix/outputs.nix — a ghc9123 dev shell (the lightweight quick-shell, since HLS/fourmolu/etc. have no GHC 9.12.3 releases yet).

Why ghc9123 needs special handling

GHC 9.12.3 ships RTS fix #25750 (underflow frame lookups in the bytecode interpreter) — the only plausibly-relevant change for this crash. But the project's pinned haskell.nix (ac987ef) does not ship GHC 9.12.3, and bumping the pin wholesale breaks the macOS dev shell (no GHC-9.12.3-compatible HLS/ghc-lib-parser releases). So:

• the stress script selects a newer haskell.nix only at build time via --override-input haskell-nix github:input-output-hk/haskell.nix/d93d6ed — flake.lock is never touched;
• the ghc9123 flake variant is inert under the pin (nothing in CI or the default shells evaluates it);
• the ghc9123 devShell is gated on pkgs.haskell-nix.compiler ? ghc9123, so nix flake check/show keep working without the override. Enter it with nix develop .#ghc9123 --override-input haskell-nix <rev>.

Notes for reviewers

• The haskell.nix override also advances iserv-proxy/nixpkgs (both crash-relevant), so a clean ghc9123 column vs a crashing ghc96 column is suggestive, not proof the GHC version is the cause. The script supports adding a ghc96-bumped arm (same GHC, bumped toolchain) to isolate that variable.
• The first ghc9123 iteration cross-compiles the GHC 9.12.3 toolchain (not in IOG's cache) and is slow; later iterations rebuild only the plutus-core library. The ghc912 mingwW64 closure is also not built by CI, so its first iteration compiles all dependencies.
• No changelog fragment: tooling/CI-investigation only, no library changes.

Pre-submit checklist:

• Branch
  • Tests are provided (if possible) — n/a, investigation tooling
  • Commit sequence broadly makes sense
  • Key commits have useful messages
  • Changelog fragments have been written (if appropriate) — n/a
  • Relevant tickets are mentioned in commit messages
  • Formatting, PNG optimization, etc. are updated
• PR
  • Targeting master unless this is a cherry-pick backport
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested