Seasons & themes: date-driven palettes and skins (#22)

[rwrife]

Closes #22

Adds a purely cosmetic seasonal dressing layer on top of the existing render pipeline. The plant's state (stage/health/streak) is untouched — seasons only add a palette + light ASCII decoration.

What's in

• New internal/season package — pure Season derived from a reference time.Time (Dec–Feb winter, Mar–May spring, Jun–Aug summer, Sep–Nov autumn). No I/O, fully unit-tested incl. month boundaries and zero-time totality.
• Render dressing (internal/render/season_render.go) — prepends a season-appropriate decoration line (snow / buds / sun rays / falling leaves), color-tinted on the color path, plain ASCII otherwise. season.None is a clean no-op.
• Flags — --season <winter|spring|summer|autumn> (alias fall) to force/preview, --no-season to disable, --holiday opt-in for gentle holiday skins (off by default, no surprise decorations).
• README — season preview + force/disable/holiday docs; roadmap + interface updated.

Acceptance criteria

• Pure date→season mapping, testable, no I/O
• Season-appropriate palette/decoration, degrades under --no-color
• --season to force, --no-season to disable
• Opt-in holiday skin gated behind --holiday
• Cosmetic only — never changes stage/health/streak (asserted in tests: TestSeasonPreservesPlantState)
• Unit tests: month→season (incl. boundaries) + state preservation
• README shows preview + force/disable
• go build ./... and go test ./... pass

Verification: gofmt -l clean, go vet ./... clean, all tests green.