Fullbleed 2.5.21: border-image tiling fixes

Fullbleed 2.5.21 fixes two border-image rendering defects. space now leaves a region empty when no complete tile fits. Center fills with a zero-width top or left edge use the opposite edge's scale, or the unscaled source when both edges are zero, preserving the intended repeat pattern.

Validation includes 1,356 Rust library tests, 11 documentation tests, 592 Python tests, and Windows/Linux CI. Eighteen raster/SVG cases pass 272 checks across native/finalized previews and independently rendered direct/compiled PDFs. Chrome printed PDFs independently confirm all 68 interior-color expectations; public 2.5.20 fails 14 cases. The public 2.5.21 wheel and a fresh crates.io consumer with svg_raster enabled reproduce all 36 verified PDFs. That Rust feature matches the SVG support included in distributed Python wheels.

The complete unchanged 1,662-fixture source corpus reports 1618 PASS, 25 FAIL, and 19 REFERENCE-DISPUTED, with no new FAIL or lost PASS verdicts. 1,662 PDFs are byte-for-byte unchanged from the retained 2.5.20 source baseline. The remaining failed gate is preserved. This corpus identifies the source candidate; public packages have separate targeted verification. These finite checks do not establish general CSS parity or PDF standards conformance.

• PyPI 2.5.21
• crates.io 2.5.21
• PDFs, previews, browser references, and runnable verification
• Complete unchanged source-corpus evidence
• Verification record
• SHA-256 checksums

Source: #67. Release preparation. Related to the open CSS tracker #54.