§H.12 step 2: session-slot table, session_find/session_register

src/starkernel/vm/session.c: kmalloc'd-at-boot slot table sized from
stadium_max_vm_count() (mirrors stadium.c's own StadiumVMQuota, not a
fixed compile-time array as originally planned -- that table was already
moved off a fixed array for the same "population isn't knowable in
advance" reason). session_boot_init()/session_find()/session_register()
implemented for real, no stubs; session_register() zeroes identity and
leaves pinned=0, matching VMIdentity's own documented default and
deferring pin policy to callers. Added session.c to Makefile.starkernel's
explicit source lists. No callers yet.

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

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

This commit is contained in: