Add per-module Haskell build foundation

Implements Steps 1-4 of per-module Nix architecture: - Add
HsModuleGraph and HsModuleNode types to represent module DAG -
Implement buildHsModuleGraph that:
  - Parses imports for each module in the transitive closure -
  Detects Template Haskell usage (language pragma or $( patterns) -
  Detects import cycles using SCC analysis - Returns Nothing (fallback
  to monolithic) if TH or cycles found
- Wire hsGraph into Target type for all builders - Populate hsGraph
for Haskell targets during analysis

Module graph is now emitted in --plan output showing per-module
dependencies. This enables the next step: per-module Nix derivations
in Builder.nix for true incremental builds.

Part of Phase 3 from the performance plan.

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