httpfs: compile remote-read optimization guards on wasm

[adsharma]

Summary

Fixes the wasm build failure in httpfs/src/httpfs.cpp:

error: unknown type name 'PersistentBlockCache'
error: unknown type name 'PrefetchZone'
error: use of undeclared identifier 'copySpanPortion'
error: use of undeclared identifier 'submitPrefetch'

HTTPFS_REMOTE_READ_OPTIMIZATIONS is 0 on wasm (http_config.h), which compiles out the PrefetchZone/PersistentBlockCache declarations in httpfs.h and the zoneFor/submitPrefetch definitions in httpfs.cpp — but readFromFile() still referenced those types unconditionally.

Changes

• Hoist copySpanPortion out of the HTTPFS_REMOTE_READ_OPTIMIZATIONS guard: it has no dependency on the guarded cache types and is needed by the L1 LRU-cache path in all builds.
• Remove the dead #else branch that declared PersistentBlockCache*/PrefetchZone* locals — those types don't exist when optimizations are off.
• Guard the optimized read layers behind HTTPFS_REMOTE_READ_OPTIMIZATIONS: the L2 (prefetch zone) + L3 (persistent disk cache) block, the write-through/submitPrefetch block after the network fetch, and the trace-log arguments referencing zone/diskCache.

Behavior on native builds is unchanged; the wasm single-threaded path now serves reads via L1 LRU + network fetch as before the optimizations were introduced.

Test plan

• em++ -fsyntax-only -D__WASM__ on httpfs.cpp: all 4 errors gone.
• Native release compile command on httpfs.cpp: clean (HTTPFS_REMOTE_READ_OPTIMIZATIONS=1 path unchanged).