Species selection (fern / cactus / bonsai / sunflower)

[rwrife]

Closes #9

Adds --species so users can pick their plant's species, each with a full stage×health art set, plus a gameplay twist for the cactus.

What changed

• New internal/species package: Kind enum (fern/cactus/bonsai/sunflower), case-insensitive Parse, and complete per-species art tables (every stage × health). Fern reuses the original art byte-for-byte, so the default is a no-op visual change.
• Cactus twist: species can override wilt thresholds. The cactus stays healthy/thirsty far longer (~10 idle days before wilting vs. ~4 for others). Implemented via a new pure plant.ComputeWith(..., HealthTuning) — growth/streak are untouched, so a species can't fake progress, only survive a drought. plant stays I/O- and species-free (tuning passed in) to avoid an import cycle.
• --species flag + config default: an explicit flag wins; otherwise the saved species in state.json is used; unknown names error with the valid list. A chosen species is remembered as the default for later flag-less runs.
• Render: Options.Species and per-GardenPlant Species select the art set cleanly by (species, stage, health).
• README: new Species section + interface/roadmap updates.

Acceptance criteria

• --species flag + config default
• At least 3 species with full stage/health art sets (4 shipped)
• Cactus survives droughts longer (different wilt thresholds)
• Render layer cleanly selects art set by species

Tests

New coverage in species, plant, and render: parse round-trips, art-table completeness/distinctness, ComputeWith-default == Compute, drought tolerance, and per-plant garden species. Full suite green; gofmt/go vet clean. Manually smoke-tested all four species, persistence, flag-less recall, and the unknown-species error.