P3: declarative ccbench_add_protocol() + move protocols under cc/ (closes #32)

[thawk105]

Summary

Closes #32.

• New cmake/ProtocolHelpers.cmake exposes ccbench_add_protocol(<name> SOURCES ... WORKLOADS ... OPTIONS ...). Each protocol's CMakeLists.txt collapses from ~70-130 lines of file(GLOB) + add_executable + add_definitions boilerplate into a single declarative call (6-19 lines).
• New cmake/Options.cmake centralises the universal -D flags as CCBENCH_* cache entries (KEY_SIZE, BACK_OFF, VAL_SIZE, etc.), replacing the ~180 directory-scoped add_definitions(...) calls duplicated across protocols. Override on the cmake command line: cmake -S . -B build -DCCBENCH_KEY_SIZE=16 -DCCBENCH_BACK_OFF=0.
• All flags are now target_compile_definitions(... PRIVATE ...) — no more directory-scope leak.
• ccbench_write_protocol_matrix() dumps build/PROTOCOL_MATRIX.md from the WORKLOADS arg of every call, so the support table in CLAUDE.md / docs/protocols.md can be refreshed mechanically.
• Bonus reorg: all 10 protocols moved from <name>/ to cc/<name>/. Binaries now land in build/cc/<protocol>/<workload>_<protocol>.exe; docs updated to match.

Completion criteria (from #32)

• Each protocol's CMakeLists.txt is ~10-20 lines
• Zero add_definitions(...) calls remain
• All protocols build (Debug+ASan and Release verified locally — 34/34 binaries)
• CI green (this PR will exercise it)

Notes / follow-ups

• The helper deliberately does not call set_compile_options() (-Wall -Wextra -Werror). The old per-protocol CMakeLists never enabled -Werror, the source tree has accumulated warnings under those flags (ermia/scan_callback, masstree_wrapper, ...), and turning it on here would mix in a separate cleanup. Left as a follow-up.
• Some <protocol>.cc files (e.g. cc/cicada/cicada.cc) are legacy standalone YCSB mains that the old CMake silently excluded — kept in tree but not in SOURCES. Could be removed in a separate cleanup.
• occ/ is still not wired into CMake; not moved under cc/ this PR.

Test plan

• cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug && cmake --build build -j succeeds locally (34 binaries)
• cmake -S . -B build-release -DCMAKE_BUILD_TYPE=Release -DENABLE_SANITIZER=OFF && cmake --build build-release -j succeeds locally (34 binaries)
• Smoke-run: ./build/cc/silo/ycsb_silo.exe -thread_num=2 -extime=1 -clocks_per_us=2100 shows both universal and silo-specific -D flags applied
• CI passes