docs(guide): document ScoreKeeper and PersistWallet

[BCook98]

Problem

The flagship leaderboard/wallet abstractions ScoreKeeper and
PersistWallet/PersistBest (kit.go ~93-99, internal/game/scorekeeper.go)
had zero mentions in GUIDE.md. Game authors were therefore steered into
the manual r.Post + KV-clobber footgun these helpers were built to eliminate:
forgetting to re-post on disconnect, never posting an abandoned-but-ticking
continuous world, and racing KV writes that clobber durable wallets at merge
time.

Fix

Add a "Bank scores the easy way: ScoreKeeper" subsection under
Scores and leaderboards in GUIDE.md documenting:

• what ScoreKeeper does and the one-keeper-on-the-room idiom (with example);
• when to use it, including continuous / never-ending (LifecycleResident) games via FlushAll from OnWake;
• cadence semantics — OnImprove vs OnChange, and the always-post-first-sighting rule;
• the three post points: live Record (always StatusFinished), FlushLeave (DNF, no-op when untracked, lower-is-better caveat), FlushAll (deterministic AccountID order, timer-free / replay-safe);
• PersistBest (MergeMax) and PersistWallet (MergeSum + MergeMax) for resume KV, with a note that these feed KV only, not the board;
• an explicit prefer this over manual r.Post + KV writes callout.

Docs only — no code changes.

Verification

• go build ./... — pass
• go vet ./... — pass
• go test . -run ScoreKeeper -v — all 6 ScoreKeeper tests pass

Every documented claim (cadence behaviour, statuses, no-op, merge rules,
deterministic order, timer-free design) was checked against
internal/game/scorekeeper.go and scorekeeper_test.go. Intra-doc anchor
links (#scores-and-leaderboards, #choosing-a-lifecycle,
#durable-state-the-per-player-kv) verified against the actual headers.

No changeset added: this is docs-only with no module-surface change, so it
should not drive a version bump / release.

For reviewers

Confirm the API descriptions match the source and that the
prefer-ScoreKeeper framing reads right alongside the existing manual
Post/End guidance.

🤖 Generated with Claude Code