fix(lifecycle-service): invalidate cached account reads after merge

[onuibeblessing2019-hash]

Closes #287

Investigation

lifecycle-service had zero caching of any kind — every getAccount call hit Horizon live. The only cache anywhere in this repo (wallet-service/src/cache.ts) is real, well-built infrastructure, but is wired into deps and never actually called from any route — unused scaffolding, and unrelated to account merges (wallet-service has no merge concept at all; merges are purely a lifecycle-service/classic-account thing). So #287's premise ("cached data is not invalidated") first needed a cache to exist before there was anything to invalidate.

Also found, while investigating: src/server.test.ts has literal syntax errors (TS1128, a stray unclosed describe block with a function illegally nested inside it) that prevent the whole file — and therefore the whole package's typecheck — from compiling, and src/account-merge.integration.test.ts references a AccountRecord type that doesn't exist anywhere and fails to even load (unrelated sac-sdk transitive-import error on top of that). Both are pre-existing and out of scope for this PR — left untouched and disclosed here rather than attempted, per the actual scope of #287.

What's here

• src/account-cache.ts (new) — createCachedAccountReader, a short-TTL (default 30s) decorator around any AccountReader. Caches both successful lookups and not-found (undefined) results, so a merged-away account correctly reads as gone once invalidated. Exposes invalidate(accountId) and invalidateAll().
• src/server.ts — POST /lifecycle/merge now invalidates both the source (merged away) and destination (balance changed) account's cache entries once the merge step is built and audit-logged, via a new isCachedAccountReader type guard (a plain, uncached AccountReader — what every existing test in this package uses — gets no invalidation call at all, since there's nothing to invalidate). Logs a new lifecycle.merge.cache_invalidated event.
• Important, disclosed nuance: this endpoint only builds the unsigned merge transaction — the client still signs and submits it separately (via wallet-service). This service has no channel to observe that later on-chain confirmation, so invalidation here is optimistic (at the point the merge is committed to, not proven complete). The short TTL is the backstop for the rare case a given merge is later abandoned or fails to land — documented in both the code comment and README.md.
• src/index.ts — production composition now wraps the real Horizon reader with the cache (previously just used the raw, uncached reader).
• README.md — new "Account read caching and invalidation on merge" section per the issue's "document the invalidation step" requirement, including the optimistic-invalidation caveat.
• Also fixes the same pre-existing workerProcessingLagSeconds is not defined" crash in packages/service-kit/src/metrics.tsalready fixed independently in #381 (#326/#327) — needed again here since this branch is based directly onupstream/dev, which doesn't have that fix yet; without it, importing @vellar/service-kit(and therefore this service's ownserver.ts`) throws at module load time.

Test plan

• account-cache.test.ts (9 new tests) — cache hit/miss, TTL expiry, not-found caching, per-account-id isolation, and the exact end-to-end scenario [medium] Fix stale cache after account merge in lifecycle-service #287 describes: a stale cache entry showing the pre-merge source-exists/dest-old-balance state, confirmed still stale without invalidation, then confirmed correct (source gone, dest's new balance) after invalidate.
• merge-cache-invalidation.test.ts (3 new tests) — exercises the real POST /lifecycle/merge route (not just the cache module in isolation): confirms both accounts are evicted after a successful merge, confirms a plain uncached reader is unaffected (no invalidation attempted, no throw), confirms a rejected merge (blockers remaining) does NOT invalidate anything.
• Full lifecycle-service suite (excluding the two pre-existing broken files above, and worker/loop.test.ts's 4 pre-existing unrelated concurrency failures, confirmed identical with my changes stashed out): 58/58 passing.
• packages/service-kit: 85/85 passing.
• npx tsc --noEmit in lifecycle-service: clean except the pre-existing server.test.ts syntax errors (identical before/after this PR).

Closes #287

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[onuibeblessing2019-hash]

closes #287