Simplify pass + code-review fixes on today's changes

[philoserf]

Summary

• Removed dice.Fixed (unused speculative scaffolding — flagged by all three cleanup review angles as having zero real callers) and its tests.
• Collapsed shipgen.structureData's aliases []string field to a scalar alias string, since only one row ever used it.
• Merged each golden-fixture / README-freshness test pair in cmd/{worldgen,chargen,sectorgen,shipgen}/golden_test.go into a single test with t.Run subtests, so each pair shares one subprocess spawn instead of two.
• cmd/worldgen/main.go: -format json now coerces a nil TradeCodes slice to [] before encoding (was rendering null for a world matching no trade code).
• cmd/worldgen/main.go: flush the buffered stdout writer before a mid-loop cli.Fatalf, so an encode error no longer silently discards already-generated records.
• internal/shipgen/design_test.go: TestDesignMurphy's core-cost assertion now pins Fuel.Cost independently (matching TestFuelMurphy's own formula) instead of reading it back off the struct under test, so it can actually catch a fuel-cost regression.

Test plan

• task check (golangci-lint + full test suite) green

[Copilot]

Pull request overview

This PR continues the repo’s ongoing cleanup/reproducibility work by removing unused dice scaffolding, simplifying a shipgen lookup table, tightening one shipgen regression test, and reducing CLI golden-test overhead while fixing a couple of worldgen JSON/IO edge behaviors.

Changes:

• Remove unused dice.Fixed and its tests.
• Simplify shipgen structure alias handling (slice → single alias) and update StructureByName accordingly.
• Improve CLI stability/perf: worldgen JSON tradeCodes now renders [] instead of null for no-code worlds; golden/README tests share a single subprocess run via subtests.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/shipgen/shipgen.go	Collapses structure alias data to a single optional alias string.
internal/shipgen/generate.go	Updates StructureByName to match against the single alias field.
internal/shipgen/design_test.go	Makes the Murphy core-cost assertion non-tautological by pinning fuel cost independently.
internal/dice/dice.go	Removes the unused Fixed roller constructor.
internal/dice/dice_test.go	Removes tests specific to the deleted Fixed helper.
cmd/worldgen/main.go	JSON output now uses [] for empty trade codes and flushes buffered stdout before fatal exit on encode error.
cmd/worldgen/golden_test.go	Merges golden + README freshness checks into subtests sharing one run.
cmd/shipgen/golden_test.go	Merges golden + README freshness checks into subtests sharing one run.
cmd/sectorgen/golden_test.go	Merges golden + README freshness checks into subtests sharing one run (with README prefix comparison).
cmd/chargen/golden_test.go	Merges golden + README freshness checks into subtests sharing one run.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.