[M3] Plant state machine

[rwrife]

The brain: turn activity (+ remembered state) into a plant state. Pure, no I/O.

Checklist

• internal/plant maps Activity -> growth stage: seed -> sprout -> leafy -> tall -> bloom
• Health modifier: healthy / thirsty / wilting based on days since last commit
• Mood/flavor text per state (short, with personality)
• Deterministic + pure: same inputs -> same PlantState
• Thresholds documented and easy to tune in one place
• Thorough unit tests covering stage transitions and wilt edges

Done when

plant.Compute(activity, state) yields the right stage/health across a table of test cases.

[rwrife]

PR up: #13 🌱

internal/plant is now the brain: plant.Compute(activity, state, now) is a pure, deterministic function mapping activity → stage (seed→sprout→leafy→tall→blooming, streak-driven + busy-day shortcut), health (healthy→thirsty→wilting by commit recency), and a short mood line. A memory floor keeps a mature plant from cratering on one quiet day (health dents first; wilting slips one stage, never below sprout). All thresholds live in one tunable block; 17 table-driven tests cover the transitions and wilt edges. gofmt/vet/build/test all green locally. Leaving the merge to next cycle so CI runs first.

[rwrife]

Merged via #13 — M3 plant state machine (activity → stage + health + mood) is in. Pure/deterministic plant.Compute, 17 table-driven tests, CI green. M4 render will consume PlantState next.