feat: migrate to system dependencies from static linking

BREAKING CHANGE: Library now requires system-installed dependencies

Major Changes:
- Convert secp256k1 from bundled static lib to system dependency
- Convert OpenSSL from bundled static lib to system dependency
- Convert curl from bundled static lib to system dependency
- Update build.sh with pkg-config detection and fallback logic
- Remove all static library extraction/building logic
- Update README.md with new dependency requirements and installation

Build System:
- Add detect_system_secp256k1() with pkg-config support
- Add detect_system_openssl() with pkg-config support
- Add detect_system_curl() with pkg-config support
- Remove secp256k1 building/extraction from ar archive
- Update CFLAGS and LIBS to use system library variables
- Clear error messages for missing dependencies with install commands

Documentation:
- Add system dependency installation for Ubuntu/Debian/CentOS/macOS
- Update all compile/link examples to include -lssl -lcrypto -lcurl -lsecp256k1
- Remove references to 'self-contained' and 'no external dependencies'
- Update integration examples throughout README

Benefits:
- Smaller library size (only internal code bundled)
- Automatic security updates via system package manager
- Standard Linux library distribution pattern
- Reduced build complexity
- Better system integration with pkg-config

Required Installation:
Ubuntu/Debian: sudo apt install libssl-dev libcurl4-openssl-dev libsecp256k1-dev
CentOS/RHEL: sudo yum install openssl-devel libcurl-devel libsecp256k1-devel
macOS: brew install openssl curl secp256k1

secp256k1.depreciated/.gitignore

@@ -0,0 +1,71 @@
+bench
+bench_ecmult
+bench_internal
+noverify_tests
+tests
+exhaustive_tests
+precompute_ecmult_gen
+precompute_ecmult
+ctime_tests
+ecdh_example
+ecdsa_example
+schnorr_example
+ellswift_example
+musig_example
+*.exe
+*.so
+*.a
+*.csv
+*.log
+*.trs
+*.sage.py
+
+Makefile
+configure
+.libs/
+Makefile.in
+aclocal.m4
+autom4te.cache/
+config.log
+config.status
+conftest*
+*.tar.gz
+*.la
+libtool
+.deps/
+.dirstamp
+*.lo
+*.o
+*~
+
+coverage/
+coverage.html
+coverage.*.html
+*.gcda
+*.gcno
+*.gcov
+
+build-aux/ar-lib
+build-aux/config.guess
+build-aux/config.sub
+build-aux/depcomp
+build-aux/install-sh
+build-aux/ltmain.sh
+build-aux/m4/libtool.m4
+build-aux/m4/lt~obsolete.m4
+build-aux/m4/ltoptions.m4
+build-aux/m4/ltsugar.m4
+build-aux/m4/ltversion.m4
+build-aux/missing
+build-aux/compile
+build-aux/test-driver
+libsecp256k1.pc
+
+### CMake
+/CMakeUserPresets.json
+# Default CMake build directory.
+/build
+
+### Python
+__pycache__/
+*.py[oc]