gallery: NVDLA sky130hd/partition_c placeholder per new per-variant rule

Per the updated /update-results skill: emit a placeholder for every (platform,
variant) slot a card should contain that doesn't have a thumb — including
missing variants within a platform that already has other thumbs. NVDLA card
now visually shows the missing sky130hd/partition_c slot (which has been a
known long-build / GP-overflow issue per CLAUDE.md, tracked as task #39).

Author: mguthaus

gallery.html

@@ -612,6 +612,7 @@ <h2><a href="https://github.com/nvdla/hw">NVDLA</a></h2>
             <a class="thumb-link" href="figures/partition_o_nangate45_79da6274.png"><img src="figures/thumbs/partition_o_nangate45_79da6274.jpg" alt="NVDLA partition_o nangate45 final"><span class="thumb-label nangate45">nangate45</span><span class="thumb-variant">partition_o</span></a>
             <a class="thumb-link" href="figures/partition_p_nangate45_79da6274.png"><img src="figures/thumbs/partition_p_nangate45_79da6274.jpg" alt="NVDLA partition_p nangate45 final"><span class="thumb-label nangate45">nangate45</span><span class="thumb-variant">partition_p</span></a>
             <a class="thumb-link" href="figures/partition_a_sky130hd_c919ca69.png"><img src="figures/thumbs/partition_a_sky130hd_c919ca69.jpg" alt="NVDLA partition_a sky130hd final"><span class="thumb-label sky130hd">sky130hd</span><span class="thumb-variant">partition_a</span></a>
+            <div class="thumb-placeholder"><div class="ph-box">pending</div><span class="thumb-label sky130hd">sky130hd</span><span class="thumb-variant">partition_c</span></div>
             <a class="thumb-link" href="figures/partition_m_sky130hd_c919ca69.png"><img src="figures/thumbs/partition_m_sky130hd_c919ca69.jpg" alt="NVDLA partition_m sky130hd final"><span class="thumb-label sky130hd">sky130hd</span><span class="thumb-variant">partition_m</span></a>
             <a class="thumb-link" href="figures/partition_o_sky130hd_79da6274.png"><img src="figures/thumbs/partition_o_sky130hd_79da6274.jpg" alt="NVDLA partition_o sky130hd final"><span class="thumb-label sky130hd">sky130hd</span><span class="thumb-variant">partition_o</span></a>
             <a class="thumb-link" href="figures/partition_p_sky130hd_c919ca69.png"><img src="figures/thumbs/partition_p_sky130hd_c919ca69.jpg" alt="NVDLA partition_p sky130hd final"><span class="thumb-label sky130hd">sky130hd</span><span class="thumb-variant">partition_p</span></a>