Add the missing YCSB entrypoint for SS2PL

[thawk105]

cc/ss2pl/CMakeLists.txt registered only bomb and tpcc, so no ycsb_ss2pl.exe was ever built and SS2PL could not be measured on YCSB at all. cmake --build <dir> --target ycsb_ss2pl.exe fails with "No rule to make target".

The pre-cmake YCSB main survived as cc/ss2pl/ss2pl.cc, but it is dead code: it is outside the build graph (the ccbench_add_protocol convention picks up <workload>_<protocol>.cc, which it does not match) and it calls the two-argument TxExecutor constructor that no longer exists. Nothing references it, so remove it rather than leave a file that cannot compile.

• add cc/ss2pl/ycsb_ss2pl.cc, modelled on cc/silo/ycsb_silo.cc and cc/ss2pl/bomb_ss2pl.cc: the shared ccbench::run<TxExecutor, TransactionStatus, YcsbWorkload> runner with the current three-argument factory
• add ycsb to the protocol's WORKLOADS
• delete the dead cc/ss2pl/ss2pl.cc
• mark SS2PL's YCSB column as supported in README.md and in both docs/protocols_ja.md and docs/protocols_en.md (each has a protocol table and a matrix snapshot; the snapshots were checked against a regenerated build/PROTOCOL_MATRIX.md, which is not committed)

Not changed on purpose:

• cc/ss2pl/util.cc is untouched. chkArg() validates the protocol-local legacy FLAGS_rratio rather than the FLAGS_ycsb_rratio the YCSB workload actually reads, but that mismatch is repo-wide (cc/silo disabled the same check) and fixing it only for this binary would make SS2PL's YCSB entrypoint behave differently from every other one.
• With ycsb_ss2pl.exe in place, YCSB aborts are counted twice: once in TxExecutor::abort() and once in the workload loop in include/ycsb.hh. That is a pre-existing issue in the shared workload header, not specific to SS2PL, and is left for a separate change.
• The per-protocol README.md and script/*.sh still refer to the old ss2pl.exe name and positional CLI. The same staleness exists under cc/silo/, so it is out of scope here.

Verified on an Intel Xeon Platinum 8468 (48 physical cores):

• cmake --build of every protocol with -DCMAKE_BUILD_TYPE=Release, -Wall -Wextra -Werror, under g++ 11.4 and g++ 12
• ycsb_ss2pl.exe -ycsb_tuple_num=1000000 -ycsb_max_ope=10 -ycsb_rratio=50 -ycsb_zipf_skew=0 -thread_num=48 -extime=5 exits 0 at 7,800,905 tps (abort rate 0.0031)
• tpcc_ss2pl.exe and bomb_ss2pl.exe are unaffected: their .text sections are byte-identical to a build of the parent commit
• the clang-format --dry-run --Werror check from .github/workflows/format.yml passes with clang-format 14

GCC 13, which CI uses, was not available on the test machine, so the -Werror result there is unverified.

This change was written with AI assistance.