Add hermetic C unit-test suite for libppd API

[rkt0209]

This PR introduces a comprehensive, hermetic C unit-test suite for the libppd public API. It adds 11 new native C unit tests alongside the pre-existing testppd framework, bringing the suite to a total of 12 passing tests.

All tests are integrated into the existing autotools make check flow, use the test-internal.h micro-framework, and embed static PPD fixtures to ensure zero runtime, network, or filesystem dependencies. Additionally, a GitHub Actions CI workflow (.github/workflows/build.yml) is included to autonomously build the project and execute this suite on ubuntu-latest.

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Test Coverage

Test File	Description
testppd	Baseline PPD parser smoke test (pre-existing).
test_ppd_localize	Tests language fallback chains and IPP reason-text lookups.
test_ppd_cache	Tests cache building and IPP-keyword reverse-mapping.
test_ppd_ipp	Tests PPD to printer-IPP attribute conversion and bi-directional name normalization.
test_ppd_mark	Tests option marking, default resets, and conflict propagation.
test_ppd_custom	Tests custom-option parameter walking and lookup functions.
test_ppd_attr	Tests attribute iteration, cursor sharing, and spec-based selection.
test_ppd_page	Tests variable and custom page size resolution, along with margin lookups.
test_ppd_conflicts	Tests UI/Non-UI constraints and installable-option conflict resolution mechanisms.
test_ppd_emit	Tests code emission, PJL preamble formatting, and the PageSize/PageRegion decision tree.
test_ppd_filter	Tests filter-data setup, hardware copies/collate decisions, and orientation transforms.
test_ppd_generator	Tests IPP-to-PPD generation, Make/Model sanitization, PDL detection, and resolution fallback.

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Verification

• Local make check passes perfectly (12/12).
• GitHub Actions CI workflow is green on ubuntu-latest.