feat: cache parsed member indexes as msgpack to skip YAML re-parse on rebuild

Warm rebuilds of virtual repos (member caches valid, virtual TTL expired)
previously re-parsed all member index.yaml files on every rebuild. With 19
Helm members totalling 14 MB, YAML parsing was 60% of merge time (~6.3s of
~9.6s). Parsing each member's YAML also produces msgpack and stores it in S3
alongside the raw index. Subsequent rebuilds load the compact msgpack and skip
YAML parsing entirely.

Before: warm rebuild ~9.6s (CSafeLoader baseline)
After:  warm rebuild ~5.9s (38% faster, merge=4.7s down from ~9.6s)

README.md

@@ -390,7 +390,7 @@ If a member is unreachable and has no cached index, it is skipped and a warning
 
 **Caching:**
 
-The merged index is cached using `min(mutable_ttl)` across all members. Each member's raw index is cached in S3 under its own remote key by the normal proxy rules; the virtual handler reuses those copies when available.
+The merged index is cached using `min(mutable_ttl)` across all members. Each member's raw index is cached in S3 under its own remote key; the virtual handler reuses those copies when available. On rebuild, each member's parsed index is also stored as a compact msgpack file (`index.msgpack`) alongside the raw YAML, eliminating the YAML parse cost on subsequent rebuilds.
 
 **Helm example:**