Retrieval vs Long Context Decision

Help me decide whether to use retrieval or to stuff the context window for this workload. Be opinionated.

Analyse:

1. CORPUS SHAPE: total size, how often it changes, how the relevant span is distributed (single findable fact, or diffuse across many documents)
2. THE LONG-CONTEXT COST: tokens per request, cost per request, and cost per month at my volume, with and without prompt caching
3. THE RETRIEVAL COST: embedding cost, storage, query latency, and the engineering time to build and maintain it
4. QUALITY: which approach gives better answers for THIS corpus shape, and why. Account for lost-in-the-middle degradation honestly.
5. VERDICT: one recommendation. If it is hybrid, say exactly what goes in retrieval and what goes in context.

Do not hedge. Pick one.

Corpus:
[DESCRIBE]
Volume:
[REQUESTS PER DAY]
Budget:
[MONTHLY]