9a7abe47

Use aeson-pretty for on-disk JSON encoding

Pretty-print every JSON artifact the mutation pipeline (and the rerun
cache) writes — redundancy.json, report.json, manifest-augmented.json,
the plugin manifest, coverage/baseline maps, kill rows, test-locations,
and the rerun report — so they are diff- and debug-friendly.  Negligible
perf cost; readers are whitespace-insensitive so nothing else changes.
The manifest golden check is unaffected (it re-normalises via jq).

Adds aeson-pretty to sydtest and sydtest-mutation-runtime (default.nix
regenerated via cabal2nix).

Agent Instructions

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