sp stats — fun store metrics

[rwrife]

Closes #32

Adds sp stats: a read-only metrics report over the index.

What it reports

• Live count + bytes; morgue count + bytes (recoverable) with purgeable-past-grace count.
• Oldest living scratch ("oldest survivor") and its age.
• Total bytes held by the store (live + morgue) — honestly labeled, no fabricated all-time counters (v0.1 keeps none on disk).
• Top-N tag breakdown (living scratches only), ranked by count then name.
• --json for scripting (color/personality-free), mirroring ls --json / doctor --json. oldest is null on empty store; tags always an array.

Structure follows existing patterns: computation in internal/store/stats.go (plain data), rendering in internal/render/stats.go, command in internal/cli/stats.go adapting store→render types (same one-way dependency arrow doctor/reap use).

Tests cover empty store, mixed live/morgue, purgeable counting, oldest-survivor selection, tag-breakdown ordering, morgue tags excluded, JSON shape + null-oldest zero-state, and full command wiring.

README updated with a sp stats section + quickstart lines.

Acceptance criteria all met. All tests + go vet green.