hyper, hyper-parquet: persistent server so hot runs are actually hot

[caetanosauer]

Fixes #936.

Problem

The shared driver (lib/benchmark-common.sh) calls ./query once per try (BENCH_TRIES) and, for daemon-backed systems, keeps the server alive across tries so tries 2..N measure hot execution. Hyper's ./query instead opened a brand-new HyperProcess on every call, so each "hot" try hit an empty buffer pool against a just-cache-dropped file — every reported hot time was actually cold. (Reported in #936; the same per-query restart was introduced for Hyper in the benchmark.sh split refactor.)

Fix

Convert both hyper/ and hyper-parquet/ to the client-server model the framework expects, mirroring umbra/:

• start backgrounds a supervisor that opens one long-lived hyperd and publishes its connection descriptor to server.endpoint. In hyper/ it also holds a keep-alive connection to hits.hyper so the buffer pool isn't torn down when each per-try ./query process exits (Hyper detaches a .hyper DB when its last connection closes).
• stop SIGTERMs the supervisor (cleanly shutting down hyperd) and waits for it to fully exit so drop_caches isn't defeated by pinned mmap pages.
• check / query / load reconnect to the persistent server via its descriptor instead of spawning their own HyperProcess. Loading through the same server also avoids briefly running two hyperd instances (each claiming ~80% RAM) during the heavy COPY.
• benchmark.sh: BENCH_RESTARTABLE=yes (there is now a real daemon whose lifecycle matters) and drop the BENCH_CONCURRENT_DURATION=0 override, re-enabling the concurrent-QPS test (a single shared server makes it meaningful again).

Net effect: the driver's cold cycle (stop → wait → drop_caches → start) gives an honest cold try 1, and tries 2..N hit the warm server = genuinely hot.

Validation

Ran the full 43-query sweep on a c7a-class x86_64 box for both directories:

Benchmark	Rows	cold(try1) ≥ hot(min t2,t3)	Median speedup	Cold→Hot total
hyper (native)	43/43	43/43	9.71×	34.4s → 4.1s
hyper-parquet	43/43	43/43	2.35×	64.4s → 22.2s

Every query on both paths shows try 1 (cold) ≥ tries 2..3 (hot), with zero anomalies; before the fix all three tries were cold and roughly equal. Native data size (~19 GB) and load time (~351s) match prior committed results. The re-enabled concurrent-QPS test on hyper-parquet produced 5.583 QPS at a 0.3% error ratio. No leaked hyperd processes after stop.

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