game: Clean up Merkle trees

[tindzk]

Until now, credit trees were retained indefinitely on both chains. This PR implements removal:

• A tree is removed once its credits are minted or its claim deadline has passed
• Each chain (People, Asset Hub) tells the other what it removed
• Each chain has a TTL sweep of its own that covers for a message that never arrived

Lifecycle of one tree

stateDiagram-v2
    state "Fully claimed" as Claimed
    [*] --> Stored: receive_credit_trees
    [*] --> Rejected: already past its deadline on arrival
    Stored --> Claimed: every leaf claimed
    Stored --> Expired: claim deadline passed, swept by bucket
    Claimed --> Queued: deletion queued
    Expired --> Queued: deletion queued
    Queued --> [*]: sent to the People chain
    Rejected --> [*]

Loading

Delivery and deletion across the two chains

sequenceDiagram
    participant PeopleOcw as People offchain worker
    participant Credits as nft-credits, People
    participant Claims as nft-claims, Asset Hub
    participant AhOcw as Asset Hub offchain worker
    participant User as Claimant

    Credits->>Claims: receive_credit_trees
    User->>Claims: claim, mints the NFT
    Note over Claims: last leaf of a tree claimed,<br/>tree removed and deletion queued
    AhOcw->>Claims: sweep_expired_trees, one bucket
    Note over Claims: trees past the claim deadline removed,<br/>deletions queued
    AhOcw->>Claims: send_tree_deletions
    Claims->>Credits: receive_tree_deletions, XCM
    Note over Credits: named roots removed
    PeopleOcw->>Credits: sweep_expired_roots, one bucket
    Note over Credits: roots past their own deadline removed,<br/>covers for a lost message

Loading

Both sweeps walk one bucket of wall-clock time per call. A tree's bucket is the UTC day its credits
were awarded in. A sweep therefore selects the trees that are due instead of reading every tree.

On Asset Hub, a tree is dropped 90 days after the credits were awarded, which is the claim deadline.
The People chain keeps the root it was built from for a further 30 days. That deadline is derived
from the Asset Hub one, so the two cannot drift apart. A credit inside its deadline always has a root
and its awards to prove against.

Changes

• nft-claims: removes a tree once it is fully claimed and once its claim deadline has passed. A
  tree that arrives past its deadline is not stored. Removals queue up and go out to the People chain
  as one message, both driven by the offchain worker.
• nft-credits: removes the roots Asset Hub reports it is finished with. It sweeps the ones no
  message ever named.
• indiv-support: holds the expiry-bucket scheme and the sweep helpers both chains drive. The
  offchain submission is shared too, so a stalled submitter behaves the same on either chain.
• Runtimes: the People chain accepts Asset Hub's unpaid deletion message. Each runtime's
  integrity test asserts that its sweeps fit a block and that a full deletion message fits the
  channel.

[andrzejSulkowski]

why WeightLimit:Unlimited and not a benchmarked weight?

[tindzk]

Limited(m) must cover the weight the destination (game chain) computes. We cannot benchmark it here. A hardcoded value would silently drop messages if the game chain weights change.

[shawntabrizi]

This code may need a storage migration depending on when it lands.

Otherwise stale storage will mess up the state transition function.