fix: make libfastfields.so link depend on cpu/cuda sub-libraries

[balbasty]

Summary

• libfastfields.so's link rule listed only $(OBJECTS) as prerequisites. Its dependency on build/lib/libfastfields-cpu.so (and, when USE_CUDA=1, build/lib/libfastfields-cuda.so) was expressed only by listing libcpu/$(CUDA_DEP) as siblings of the link target under the recipe-less lib: goal — not as a real prerequisite of the link rule itself.
• Under make -j, prerequisites of lib: have no ordering guarantee and can run concurrently, so the link step could start (and finish) before make -C cpu install had produced build/lib/libfastfields-cpu.so, failing with ld: cannot find -lfastfields-cpu.
• Fix: add $(BUILDDIR)/lib/libfastfields-cpu.$(SOSUF) and $(CUDA_DEP) as real prerequisites of the $(BUILDDIR)/libfastfields.$(SOSUF) rule, so make -j cannot schedule the link before they exist. The recipe now references $(OBJECTS) explicitly instead of $^, since $^ would otherwise also pull the newly-added .so prerequisites into the link command's positional arguments (redundant with the existing -lfastfields-cpu/$(CUDA_LDFLAGS) flags).
• Checked cuda's submake for the analogous issue per the issue's suggestion: same pattern ($(CUDA_DEP) was only a sibling, not a real prerequisite), fixed in the same rule. Both cpu-lib's and cuda-lib's own internal install/libcpu/libcuda rules already declare correct real (non-sibling) prerequisites, so no further changes were needed there.

Closes#56

Verification

Ran from a clean build/ each time (rm -rf build), CPU path only (USE_CUDA defaults to 0, matching how CI builds this repo today).

Before the fix, a clean make -C . all CXX=clang++ -j4 reproduces the exact failure from the issue:

clang++ -std=c++11 -O3 -ferror-limit=1 -ftemplate-backtrace-limit=0 -shared -fPIC -Wl,-soname,libfastfields.so -Wl,-rpath,'$ORIGIN'/../lib \
-L./build/lib -lfastfields-cpu \
-o build/libfastfields.so build/distance.o build/posdef.o build/resize.o build/restrict.o build/splinc.o build/pushpull.o build/reg_field.o build/reg_flow.o
/usr/bin/ld: cannot find -lfastfields-cpu: No such file or directory
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [Makefile:187: build/libfastfields.so] Error 1

(the top-level link ran before make -C cpu install had finished copying libfastfields-cpu.so into build/lib/).

After the fix:

• make -C . all CXX=clang++ -j4 from a clean build/ — 2 consecutive clean runs, both succeed, both produce build/libfastfields.so and build/lib/libfastfields-cpu.so, no cannot find -lfastfields-cpu / no Error markers in either log.
• Plain non-parallel make -C . all CXX=clang++ from a clean build/ — also succeeds, same artifacts produced, no errors.

$ tail -3 after-j4-run1.log
-L./build/lib -lfastfields-cpu \
-o build/libfastfields.so ./build/distance.o ./build/posdef.o ./build/resize.o ./build/restrict.o ./build/splinc.o ./build/pushpull.o ./build/reg_field.o ./build/reg_flow.o
make: Leaving directory '/home/user/fastfields-lib'
$ tail -3 after-j4-run2.log
-L./build/lib -lfastfields-cpu \
-o build/libfastfields.so ./build/distance.o ./build/posdef.o ./build/resize.o ./build/restrict.o ./build/splinc.o ./build/pushpull.o ./build/reg_field.o ./build/reg_flow.o
make: Leaving directory '/home/user/fastfields-lib'
$ ls -la build/libfastfields.so build/lib/libfastfields-cpu.so # after both -j4 runs and the non-parallel run
-rwxr-xr-x 1 root root 55448 build/libfastfields.so
-rwxr-xr-x 1 root root 163008960 build/lib/libfastfields-cpu.so

• make -n dry-run sanity check that the link target now lists the cpu .so as a prerequisite
• Clean make -C . all CXX=clang++ -j4 — before: reproducibly fails; after: 2/2 clean runs pass
• Clean non-parallel make -C . all CXX=clang++ — passes
• No standalone tests at this level (per CLAUDE.md); correctness remains gated by fastfields-cpu-lib's test suite, unaffected by this change

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Generated by Claude Code

[github-actions]

Submodule staleness (non-blocking -- fastfields-lib#15)

submodule	pinned commit	tracks	status	behind by
cpu	bf57a02	fastfields/fastfields-cpu-lib@main	up to date	0
cuda	b8e5e9c	fastfields/fastfields-cuda-lib@main	up to date	0