Fix null-terminated string handling in DAT files

[redtoad]

Summary

• Replace the broken NullString.Unpack method with a String() method that strips at the first null byte on access. The old Unpack consumed the
  entire remaining buffer, breaking parsing of any struct with fields after the name.
• Use NullString type for Name fields in SaveinfoFile, SoldierData, and BaseData so null-terminated strings are handled correctly throughout.
• Fix two pre-existing round-trip issues: name the anonymous _ field in SaveinfoFile so its value is preserved on pack, and widen BaseData.Active
  from int8 to int32 to cover the full 292-byte record.

Test plan

• NullString.String() strips at null byte and handles missing null
• NullString works in multi-field structs (new test)
• Multi-field NullString round-trips correctly (unpack → pack = identical bytes)
• SaveinfoFile name parsing returns clean strings ("Test", not "Test\x00...")
• SaveinfoFile round-trip produces identical binary output
• SoldierFile round-trip produces identical binary output (250 records, 17 KB)
• All existing tests pass

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes parsing/round-tripping of null-terminated fixed-size strings in X-COM DAT files by removing a broken custom unpacker and switching affected DAT structs to a NullString representation that trims at the first NUL on access.

Changes:

• Replace NullString.Unpack with NullString.String() and migrate name fields to NullString (soldiers, bases, save metadata).
• Fix SAVEINFO.DAT round-tripping by making the missdat flag field exported and preserving the full 40-byte record.
• Adjust BASE.DAT record layout by widening the Active field storage to cover the full 4-byte value.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
savegame/soldiers.go	Use NullString.String() for soldier names and empty checks.
savegame/metadata.go	Return save title via NullString.String().
savegame/bases.go	Use NullString.String() for base names and presence checks.
internal/types.go	Remove broken unpacker and add NullString.String() truncation behavior.
internal/types_test.go	Update and expand tests for NullString.String() and multi-field round-trip.
internal/geoscape/soldier_dat.go	Store soldier name as internal.NullString in SOLDIER.DAT struct.
internal/geoscape/soldier_dat_test.go	Adapt tests for NullString name handling.
internal/geoscape/saveinfo_dat.go	Store save name as NullString and export missdat flag field.
internal/geoscape/saveinfo_dat_test.go	Add tests for SAVEINFO.DAT name parsing and round-trip.
internal/geoscape/base_dat.go	Store base name as NullString and widen Active storage to 4 bytes.

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