fix(store): bound jemalloc downloads

[imbajin]

Purpose

Store startup downloads an optional jemalloc library before launching Java. A mirror that accepts a connection but stops responding could hold startup indefinitely. The download now allows 5 seconds to connect and 30 seconds in total, then follows the existing fallback and starts without adding the downloaded library to LD_PRELOAD.

Changes

• Bound the optional transfer while preserving checksum-verified cache reuse and atomic replacement.
• Keep an existing destination and remove partial downloads after a transfer failure.
• Document the startup timeout and cover cache, timeout and cleanup behavior in the existing launcher regression suite.

Validation

The existing shell regression suite passed, including the new cases. With a real HTTP connection that never responded, the old transfer remained blocked; the new transfer timed out after about 30 seconds, cleaned its temporary file and took the optional fallback. Formatting and a clean Java 17 reactor compile passed. Three independent source reviews found no actionable issue.

The same org branch passed its selected CI, including launcher contracts, Server/RocksDB/HStore tests, container build and deployment checks, Compose smoke and platform compatibility. Local reproduction used scripts and loopback HTTP; the original release-container and Kubernetes acceptance scenarios remain separate follow-up checks.

Org validation: hugegraph#287

Documentation

Doc - Done: hugegraph-store/README.md.

Paired website documentation: ASF #517. Coordinate the documentation merge with this code change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.30%. Comparing base (7644566) to head (945b0e5).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3310      +/-   ##
============================================
+ Coverage     52.29%   52.30%   +0.01%     
+ Complexity     6052     2271    -3781     
============================================
  Files           762      762              
  Lines         67399    67399              
  Branches       9138     9138              
============================================
+ Hits          35244    35251       +7     
  Misses        28601    28601              
+ Partials       3554     3547       -7     

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