fix(gnogenesis): decode auth/accounts as gnoland.GnoAccount

[gfanton]

Summary

Follow-up to #5511. Addresses one specific case of the "Still open" item:

queryAccountAtHeight silent nil — all error paths return nil with no indication; flaky RPC → wrong sequence metadata.

This isn't flaky RPC. The parse fails deterministically on any real gno.land chain.

The bug

auth/accounts/<addr> is handled by authHandler.queryAccount, which marshals whatever acck.GetAccount returns. On gno.land that's a *gnoland.GnoAccount:

type GnoAccount struct {
    std.BaseAccount
    Attributes BitSet `json:"attributes" yaml:"attributes"`
}

The wire format is therefore {"BaseAccount":{...},"attributes":...}. The previous parser tried two paths:

1. A wrapper struct containing only BaseAccount — amino's strict-field policy rejects the unknown "attributes".
2. A bare std.BaseAccount — same problem, plus "BaseAccount" is also unknown to it.

Both error. The function silently returns nil, and the caller in FetchTxs defaults accNum to 0 for the rest of the run. Every SignerInfo entry in the generated hardfork genesis claims the same accNum.

Impact

Affects every hardfork generated with gnogenesis fork generate against a real gno.land chain:

• If the chain has more than one balance entry and balances[0].Address is not a tx signer, target boot fails loudly via the validateSignerInfo preflight added in feat(gnoland): chain hardfork mechanism v3 #5511: "account number 0 already assigned to X, cannot reassign to Y".
• If balances[0].Address happens to be the only signer (single-account testbeds), the wrong accNum=0 still propagates into target account state via the SignerInfo force-set in loadAppState, just not loudly. That's silent corruption.

The hf-glue end-to-end run in #5511 didn't surface this because that testbed only validates against rpc.gno.land data; the dominant signer there happened to land at balance index 0.

Fix

Decode the response as gnoland.GnoAccount directly. Same pattern as gnoclient.QueryAccount (gno.land/pkg/gnoclient/client_queries.go:59-65). The package is already imported in this file, so there is no architectural cost. The std.BaseAccount fallback is dead — contribs/gnogenesis is a gno.land tool — and is removed.

7 lines replacing 11.

Discovery

Caught by an end-to-end transition harness driving source → halt → fork generate → target boot → replay-report assertion, against a multi-balance source chain. The handler-side query reproduction is mechanical: any gnogenesis fork generate -source http://... against a gno.land chain with signed historical txs hits this.

AI disclosure

Diagnosed and patched with Claude Code.

[Gno2D2]

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[moul]

looks good, but ideally should be unit tested.

[codecov]

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✅ All modified and coverable lines are covered by tests.

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[aeddi]

Can you resolve the conflicts, address the comments then merge this?