fix: update OLD_WASM_HASH to cover sites stored under previous contract key

Users with Delta sites stored by the previous release (WASM hash
b92da83d…) were stuck on a permanent "Loading..." screen after the
V7 delegate upgrade republish. Root cause: adding a new
`DelegateRequest` variant to `common/src/state.rs` perturbed the
contract WASM hash (b92da83d → 53e3395f) because the contract
depends on `delta-core`. `restore_known_sites` correctly detected a
WASM upgrade for sites that had a persisted `contract_key_b58`, but
records restored from legacy delegates that pre-date b82d3bc have
`contract_key_b58 = None` and fell through to the one-hop
`OLD_WASM_HASH` fallback. That constant was still pinned to a very
old hash (1188d108…, commit 2e664c3) and had never been updated
through subsequent releases, so the fallback GET went to a contract
key where no data ever lived.

Update `OLD_WASM_HASH` to the previous release's hash
(b92da83d…) so legacy-record sites can find their state, and add
a prominent doc comment explaining that this constant MUST be
updated before every release whose commit touches `common/` or
`contracts/site-contract/`, including seemingly-unrelated changes
like enum additions that happen to rebuild the contract.

Delegate and contract WASMs are unchanged by this commit; only the
UI is rebuilt. No migration entry needed.

[AI-assisted - Claude]

Author: sanity