Add HyperLogLog support for COUNT (NIP-45)

[alltheseas]

Closes #178

Summary

• Add HLL sketch (256 registers, precision 8) to COUNT responses for single-tag filters (#e, #p, etc.), enabling clients to merge counts from multiple relays without double-counting pubkeys
• Algorithm is a mechanical port of go-nostr nip45/hyperloglog: extract 8 bytes from pubkey at a filter-derived offset, use first byte as register index, count leading zeros in remaining 56 bits
• COUNT responses now include a "hll" field (512-char hex string) when the filter has exactly one tag with one value

Files

File	Action	What
src/HyperLogLog.h	NEW	Self-contained HLL struct (addPubkeyBytes, encodeHex, clz56)
src/DBQuery.h	MODIFY	Add hllOffset/hll fields, computeHllOffset(), update registers during scan
src/QueryScheduler.h	MODIFY	Pass hllHex string through onComplete callback
src/apps/relay/RelayReqWorker.cpp	MODIFY	Include "hll" field in COUNT JSON response
src/apps/relay/RelayNegentropy.cpp	MODIFY	Update onComplete signature (ignore new param)
test/hll_reference.py	NEW	Python oracle (exact go-nostr port) for test vector generation
test/hll_unit_test.cpp	NEW	Standalone C++ test validating against Python oracle
test/nip45_hll_test.py	NEW	E2E websocket test (5 test cases)
test/cfgs/nip45HllTest.conf	NEW	Test relay config

References

• NIP-45 spec: https://github.com/nostr-protocol/nips/blob/master/45.md
• Tracking: NIP-45: Counting Results (COUNT verb) nostrability/nostrability#291
• Reference impl: https://github.com/nbd-wtf/go-nostr/tree/master/nip45/hyperloglog

Test plan

• Python oracle generates test vectors for known pubkeys
• C++ unit test passes against Python oracle (g++ -std=c++20 -I src test/hll_unit_test.cpp)
• E2E websocket test against live strfry on Linux
• Full make build on Linux

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