Fix quality-score link after streaming refactor

[andreaborio]

What

make -C gguf-tools quality-score fails to link on current main (80ebbc3).

The streaming refactor made ds4.o depend on ds4_distributed.o
(ds4_session_* → ds4_dist_session_*) and ds4_ssd.o
(ds4_engine_open/close → ds4_ssd_*), but the quality-score target still
links only ds4.o plus the backend object. The link fails with undefined
ds4_dist_* / ds4_ssd_* symbols on both the Metal and CUDA branches, and no
score_official binary is produced.

Fix

Add ds4_distributed.o and ds4_ssd.o to QUALITY_TARGETS and QUALITY_OBJS
on both branches, matching the CORE_OBJS set that ds4 and ds4-server
already link.

Reproduce / testing (per CONTRIBUTING.md)

Build-only change to a tools target; no inference backend source is touched, so
the correctness (ds4_test) and speed (ds4-bench) regression tracks are not
affected by this change. The failure mode it addresses is the broken link.

Machine / backend: macOS, Metal, Apple M5 Pro.

Before:

$ make -C gguf-tools quality-score
...
ld: Undefined symbols for architecture arm64:
  "_ds4_dist_session_create", referenced from: _ds4_session_create in ds4.o
  "_ds4_ssd_cache_experts_for_byte_budget", referenced from: _ds4_engine_open in ds4.o
  "_ds4_ssd_memory_lock_acquire", referenced from: _ds4_engine_open in ds4.o
  ... (ds4_dist_* and ds4_ssd_*)
clang: error: linker command failed with exit code 1
make: *** [quality-score] Error 1

After:

$ make -C gguf-tools quality-score
cc -I.. -o quality-testing/score_official quality-testing/score_official.c \
   ../ds4.o ../ds4_distributed.o ../ds4_ssd.o ../ds4_metal.o \
   -lm -pthread -framework Foundation -framework Metal
$ ls gguf-tools/quality-testing/score_official     # built

CUDA branch changed symmetrically but not built here (no CUDA box). It
mirrors the CUDA CORE_OBJS already used to build ds4-server, so it should
link there too — worth a confirm on a CUDA machine.

Scope

Only the two existing Makefile branches (Metal, CUDA). ROCm quality-score
wiring is pre-existing / separate and left untouched.