fix(ninja): compile shared libraries with PIC

[Abd-ullah2001]

Summary

• Default shared-library compile commands to -fPIC.
• Preserve an explicit PIC policy while ensuring a later PIE flag cannot
  replace PIC for a shared-library target.
• Cover both the Ninja manifest and compilation database output with regression
  tests.

Why

The Ninja backend already links shared_library targets with the platform's
shared-library rule, but their source objects could be compiled without
position-independent code. On ELF and common cross toolchains, that can make
the final link fail with relocation errors asking for the objects to be rebuilt
with -fPIC.

PIC and PIE are also not interchangeable here: PIE output is intended for
executables. The resolver now considers flag order and makes sure the last
effective policy for a shared-library compile is PIC, unless the project has
explicitly selected a PIC policy itself.

Validation

• python -m pytest tests/ebuild/test_ninja_backend.py -q --tb=short — 13 passed, 1 skipped
• python -m pytest tests/unit/test_ninja_backend.py -q --tb=short — 16 passed
• ruff check ebuild/build/ninja_backend.py tests/ebuild/test_ninja_backend.py — passed
• Changed-file mypy check — no issues in 2 source files
• python -m build — sdist and wheel built successfully

The full suite currently finishes with 9 failures, 681 passes, and 1 skip. All
nine failures reproduce on upstream master and come from the existing missing
PackageRecipe.to_dict() method in index synchronization; that separate issue
is being addressed in #119. Full-repository Ruff also reports four existing,
unrelated findings being addressed in #122.