§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>

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