Set the memory allocation of the ckBTC archive canister

Proposal to change the memory allocation of the ckBTC archive canister

Target canister: nbsys-saaaa-aaaar-qaaga-cai

New memory allocation: 2_147_483_648

Previous ckBTC archive proposal: https://dashboard.internetcomputer.org/proposal/143835

Motivation

Set the memory_allocation of the ckBTC archive canister from its current value of 0 (best-effort) to 2_147_483_648 (2 GiB). The current value can be obtained by calling the canister_status endpoint of the NNS root canister.

With a best-effort allocation, every memory growth on a subnet above its storage reservation threshold has to reserve cycles up front, and is rejected once the canister's reserved_cycles_limit is exhausted. With a reserved allocation the amount charged is the change in max(allocation, usage), which stays constant while usage is below the allocation, so growth within the allocation reserves nothing and cannot be rejected on those grounds.

The canister currently uses 924 MiB, so a 2 GiB allocation leaves 1,124 MiB of headroom (2.22x current usage). Block archiving is currently disabled, so the archive receives no new blocks and its usage is presently static; a 1 GiB allocation would already cover it (the archive has no pre_upgrade hook, and its post_upgrade merely reads the stable-structure headers, so even an upgrade allocates essentially nothing). The larger 2 GiB allocation is chosen so the reservation still holds if block archiving is re-enabled in the future and the archive resumes growing, avoiding a follow-up proposal under possible subnet pressure.

Setting an allocation is itself charged a reservation proportional to the subnet's storage saturation at the time. The fiduciary subnet is currently far below its 750 GiB reservation threshold, so this proposal reserves no cycles; the same change made while the subnet is under storage pressure would demand a large reservation and could be rejected outright. A reserved allocation is charged whether or not it is used, at roughly 26.2T cycles per GiB per year on this 34-node subnet - about 52.4T cycles per year for the full 2 GiB. The canister is already billed for the memory it uses, since storage is charged on max(allocation, usage), so the incremental cost introduced by this proposal is only that of the roughly 1,124 MiB of currently-unused headroom being reserved: about 28.7T cycles per year.