Apply remaining #418 performance optimizations on top of #417

[kibertoad]

Rebases the still-relevant parts of #418 onto current main, which now includes #417.

The two bug fixes #418 originally carried already landed via #417:

• falsy value caching in InMemoryCache.getMany / InMemoryGroupCache.getManyFromGroup (undefined check instead of a truthy check)
• InMemoryGroupCache reads no longer inserting empty groups into the LRU

So this keeps only the performance and code-quality changes from #418:

• AbstractFlatCache / AbstractGroupCache: resolve the cache key once and reuse it across getInMemoryOnly / getAsyncOnly / get (new private *Resolved methods) instead of re-resolving per call. Layered on top of Fix race conditions and correctness issues in caching layers #417's write-back fencing.
• getMany merge appends async results into the in-memory result array in place (always freshly allocated) instead of spreading into a new array.
• Loader / GroupLoader: build cache entries with an explicit loop, merge cached and loaded values with concat (cachedValues may be owned by a user-implemented async cache), hoist the dataSource local.
• AbstractRedisCache: precompute keyPrefix. RedisGroupCache: precompute the group index prefix and per-group entry prefix.
• unique(): early return for arrays of 0-1 elements.

Regression tests added: falsy-caching path at the Loader / GroupLoader level, and the unique() early return.

tsc --noEmit is clean and the full suite (330 tests, Redis included) passes. Biome reports only the 8 formatting diagnostics already present on main; this change adds none.

Supersedes #418.

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⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: b1b01811-9880-4646-9464-255d3bef0a89

📥 Commits

Reviewing files that changed from the base of the PR and between 3a0826b and 6528633.

⛔ Files ignored due to path filters (1)

• pnpm-lock.yaml is excluded by !**/pnpm-lock.yaml

📒 Files selected for processing (12)

• lib/AbstractFlatCache.ts
• lib/AbstractGroupCache.ts
• lib/GroupLoader.ts
• lib/Loader.ts
• lib/redis/AbstractRedisCache.ts
• lib/redis/RedisGroupCache.ts
• lib/util/unique.spec.ts
• lib/util/unique.ts
• package.json
• pnpm-workspace.yaml
• test/GroupLoader-main.spec.ts
• test/Loader-main.spec.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch merge-418-optimizations

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