feat: meta-progression (unlockable ships + loadouts)

## Summary
Persistent meta-progression layer between runs.

## Scope
- Save/load meta state (LÖVE2D save directory)
- Unlockable ship chassis (3 ships to start)
- Unlockable starting loadouts
- Sector unlock tracking (deeper sectors open as you reach them)

## Depends on
- #1 (scaffold)
- #3 (art for ship variants)

Start by reviewing the scaffold from #1 and the ship-variant art from #3, then inspect how the LÖVE2D save directory is handled. Define and implement persistent meta state for ship chassis, starting loadouts, and reached sectors; done means state survives runs and unlocks become available as specified.

Written by the indexing model from the issue text.