slides: fix title page spacing, enable LIP6 and Sorbonne

- Reserve a left margin in the partner logo grid so a full grid stays
  clear of the affiliations.
- Let the space above the title shrink when the title page overflows,
  so the logos keep a bottom margin equal to the side margins.
- Enable the LIP6 and Sorbonne Université partners in the example deck.
- Note that author and institute blocks must stay plain: table and
  flushleft wrappers add vertical space.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Slides/slides-partners.sty

@@ -88,7 +88,8 @@
   {
     \int_set:Nn \l__slides_partner_count_int
       { \seq_count:N \l__slides_partner_files_seq }
-    \dim_set:Nn \l__slides_partner_width_dim {\linewidth}
+    % Keep a margin on the left so a full grid stays clear of the affiliations.
+    \dim_set:Nn \l__slides_partner_width_dim {.82\linewidth}
     \dim_set:Nn \l__slides_partner_maxheight_dim {.24\paperheight}
     \dim_set:Nn \l__slides_partner_xgap_dim
       { \fp_to_dim:n {min(.04 * \dim_to_fp:n {\linewidth},
@@ -153,7 +154,7 @@
               {
                 \int_compare:nNnT {##1} > {1}
                   { \vskip \l__slides_partner_ygap_dim }
-                \hbox_to_wd:nn {\l__slides_partner_width_dim}
+                \hbox_to_wd:nn {\linewidth}
                   {
                     \hfil
                     \int_step_inline:nnn {##1}