dig-node: wallet_coins answers zero for every CAT while unsynced — the mirror image of #2879

[MichaelTaylor3d]

Task

WalletBackend::wallet_coins (dig-node crates/dig-wallet/src/sage/rpc.rs) answers zero for
any CAT while the local replica is unsynced, because its fallback branch bails out rather than
reading:

Source::Fallback => {
    // CAT asset attribution while syncing needs puzzle uncurrying (follow-on), so
    // the syncing-fallback CAT set is empty until the DB converges.
    if asset_id.is_some() {
        return Ok(Vec::new());
    }

This is the mirror image of #2879: not a holding the user does not have, but a holding they
DO have reported as nothing. It reaches get_spendable_coin_count and the Sage-parity token
surface, so a real $DIG holder on an unsynced node reads as holding none.

Context

Found while sweeping rpc.rs for #2879's defect shape (PR #224). It is a
different defect class from #2879 — an under-report, not an over-report — which is why it was
logged rather than folded into that PR.

The blocker named in that comment no longer exists. #2879 established that "puzzle uncurrying"
is unnecessary: a CAT coin's asset is identified by where the coin sits, not by uncurrying it.
digstore_chain::cat::cat_puzzle_hash(owner_p2_hash, asset_id) gives the puzzle hash a given
asset's coins live at, so a hint read can be filtered to one asset with no uncurrying at all.
PR#224 added exactly that filter as WalletBackend::asset_scoped_fallback_coins; this call site
takes an arbitrary asset_id: Option<&str> rather than a BalanceAsset, so it needs the hex
parsed to Bytes32 first — the only real work here.

See also the canonical skill, "A hint is not an asset — where a CAT coin lives".

Scope

Wire the fallback CAT arm to a filtered hint read. Note the blast radius is wider than #2879's:
wallet_coins feeds spend-input selection, so this changes what a spend can select while
unsynced — worth a careful look at whether an unconfirmed/unattributed coin should be selectable,
which is a judgement #2879 did not have to make.

Evidence

A test proving a $DIG holder on an unsynced replica sees their coins, plus a control proving a
foreign CAT hinted to the same address does not appear as theirs (both directions, as in #224).

---

Orchestrator ticket: https://github.com/DIG-Network/dig_ecosystem/issues/205

Moved from DIG-Network/dig_ecosystem#2902 so this repo's own PR can close it with Closes (CLAUDE.md §1.3).
GitHub refuses a private-to-public issue transfer, so the body was copied and comment history
stayed on the original.