feat: add initial

Signed-off-by: kjuulh <contact@kjuulh.io>

todos/SQ-018-s3-read-fallback.md

@@ -0,0 +1,29 @@
+# SQ-018: S3 Read Fallback
+
+**Status:** `[ ] TODO`
+**Blocked by:** SQ-017
+**Priority:** Medium
+
+## Description
+
+When a consumer requests an offset from a trimmed segment, fetch it from S3 instead.
+
+## Files to Create/Modify
+
+- `crates/sq-storage/src/engine.rs` - update read path with S3 fallback
+- `crates/sq-storage/src/object_store/reader.rs` - download + decompress segment from S3
+
+## Behavior
+
+1. Read path checks local WAL first
+2. If segment not found locally, check if it's in S3 via the index
+3. Download segment from S3, decompress (zstd)
+4. Read messages from the downloaded segment
+5. Optionally cache downloaded segment locally for subsequent reads
+
+## Acceptance Criteria
+
+- [ ] Write messages, ship to S3, trim locally, read from S3 -> messages correct
+- [ ] CRC validation on S3-fetched data passes
+- [ ] S3 fetch failure: returns appropriate error
+- [ ] Performance: subsequent reads of same trimmed segment use local cache