feat(da): add Go DA interface and in-memory mock for fibre

[walldiss]

Adds a fibremock package with a Go DA interface and in-memory mock implementation for fibre integration.

Migrated as-is from celestiaorg/x402-risotto#16 for integration into the fiber branch. Colleagues can rebase/adapt as needed.

Contents

• pkg/da/fibremock/fibre.go — DA interface (Upload/Download/Listen) with design assumptions documented
• pkg/da/fibremock/mock.go — MockDA: memory-bounded (LRU eviction), configurable retention, namespace filtering
• pkg/da/fibremock/mock_test.go — 9 tests covering all paths

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[julienrbrt]

Thanks!