[Track] Submit path fidelity: incremental deserialize/validate/apply expansion

[SMC17]

Goal

Extend submit from minimal path toward higher-fidelity behavior without sacrificing deterministic errors.

Deliverables

• Add one transaction-shape increment at a time.
• Define explicit deterministic RPC errors for each new failure class.
• Gate each increment with unit tests and Gate C fixture contracts.

Acceptance

• New submit shapes include both success and negative contract fixtures.
• No placeholder-success behavior for implemented shapes.
• Regression tests cover account mutation safety and sequencing behavior.

Linked Epic

• [Vision Epic] Agent-native XRP Ledger daemon in Zig: execution program #4

[SMC17]

Child delivery queue: #17 (good first issue), #18 (expert-only). Suggested order: #17 -> #18.

[SMC17]

New child issues for this track: #33 (submit success-state contract), #34 (submit amount/zero-value boundary errors).