BugReport: cover fetch range accounting in the cache

[michalhosna]

Two tests in their own target, one per rule:

• a fetch stream that FINs with no object bounds its non-existence inference at the Largest Location in FETCH_OK; the range past that Largest resolves against upstream;
• a whole-group fetch (End Location object 0, §10.12.1) on a track the cache does not hold keeps every object it delivered.

Problems:

• The cold path in MoqxCache::fetch passes the spec-encoded end {0,0} straight into FetchWriteback and FetchRangeIterator. The warm path normalizes it to an exclusive {1,0} first. The iterator ends where it starts, so each later object's gap marking wipes the previous one. All three objects are cached, only the last one is served.
• FetchWriteback::endOfFetch marks gaps up to the iterator's end and never consults the upstream FETCH_OK End Location. The TODO in fetchUpstream already names this.

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