Language-guided robot navigation under partial observability requires robots to reason over spatial-semantic evidence accumulated across observations, particularly when goals involve spatial relations between objects. This paper presents $S^3$-Nav, a structured spatial-semantic decision-making framework that integrates online mapping, persistent object--relation memory, and goal-conditioned navigation. Using RGB-D observations and SLAM, $S^3$-Nav maintains a persistent object--relation memory containing object records and confirmed relation records alongside an evolving navigation context containing exploration frontiers, without requiring a pre-existing map. A goal-conditioned evidence interface selects task-relevant evidence from this structured state and constructs a capacity-controlled decision view as part of the high-level policy input for exploration, target approach, and stopping decisions. The persistent memory remains available for subsequent reasoning and verification even when some of its records are excluded from the decision view. To ensure grounded execution, provisional policy decisions are independently validated against persistent object--relation memory and current geometric evidence before physical execution or task termination. Evaluations across multiple environments and object- and relation-goal tasks show improved closed-loop navigation reliability and reduced language-model token consumption with capacity-controlled evidence exposure under the evaluated inference configuration.

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