The tree now verifies on the hosted CI again

The heaviest single obligation drops from ~57 GB of memory to under 9 GB, which is what lets the standard GitHub-hosted runner build the full tree and run the axiom sweep. Nothing about the verified statements changed.

Newly formalized since v0.2.0 (development snapshot 7e91dceb532f13d0df40deaca9a1a38203df29c5, 2026-08-30):

• lem:cornerpivot (vii)'s certificate tiers
• lem:spectator's derivation tier
• The entry-read tier