bug: Ninja shared-library links omit toolchain linker flags and sysroot #139

What happened?

The Ninja backend omits toolchain linker flags and sysroot when linking shared-library targets. Executable and test targets include these settings, but shared libraries do not.

This can silently ignore linker checks such as -Wl,--no-undefined and omit the configured --sysroot.

The fix and regression tests are submitted in PR #125:
#125

Expected behavior

Shared-library links should include toolchain linker flags and sysroot before target-specific linker flags, matching executable targets. Neither the toolchain nor target flag lists should be mutated.

Reproduction steps

1. Use the code before the fix in PR fix(ebuild): preserve toolchain linker flags for shared libraries #125.
2. Configure a shared_library target with toolchain linker flags containing -Wl,--no-undefined and a sysroot.
3. Generate the Ninja build manifest.
4. Inspect the shared-library ldflags: the toolchain linker flag and --sysroot are missing.
5. On Linux/GCC, build a shared library containing an unresolved external symbol. The requested --no-undefined check is not enforced.

PR #125 adds manifest tests and a Linux/GCC CLI regression test for this behaviour.

Version or commit

8b623d5 (baseline before the fix in PR #125)

Environment

Linux, Python 3.12, Ninja backend, GCC.

Exact Linux distribution, GCC version and Ninja version were not recorded.

Real Windows/macOS linking and cross-compilation with an SDK were not tested.

Logs or diagnostics

Historical verification recorded for PR #125:

pytest tests/unit/test_shared_library_toolchain.py -q

All three regression cases failed against the original backend and passed with the fix.

Full-suite result: 9 failed, 672 passed, 3 skipped.
The nine failures were pre-existing test_index_sync.py failures involving a missing PackageRecipe.to_dict method, also reproduced on the baseline.

These are recorded verification results, not a fresh test run.

Submission checks

• I searched existing issues for a duplicate.
• This report contains no security-sensitive details.