§H.12 step 3: session_set_pinned/session_is_pinned pin-authority choke point

session_is_pinned() reads Session.pinned directly (authoritative, no
Stadium re-derivation); session_set_pinned() writes both Session.pinned
and the mirrored STADIUM_FLAG_PIN bit on the session's own patron cell,
keeping Stadium's internal eviction/admission logic (which must stay
self-contained) in sync without it calling back into session.c.

Added Session.stadium_cell (index into stadium_cells()) -- necessary
plumbing not in the original H.2 field list; the choke point can't reach
the right patron header without it. Moved STADIUM_FLAG_PIN from a
stadium.c-private #define to stadium.h (public) so session.c can
reference it without a duplicate definition.

Verified 3-arch boot to ok> (amd64/aarch64/riscv64).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/starkernel/vm/stadium.c

@@ -335,7 +335,8 @@ uint64_t stadium_density(size_t cell_index) {
     return header->heat / (uint64_t)header->mass;
 }
 
-#define STADIUM_FLAG_PIN 0x01u
+/* STADIUM_FLAG_PIN moved to stadium.h (FABRIC-3.md §H.12 step 3) -- see its
+ * doc comment there for why. */
 
 static int bitmap_get(size_t cell_index) {
     return (stadium_bitmap[cell_index / 8u] >> (cell_index % 8u)) & 1u;