Add spawn immunity check to canBuild for structure spawning

[coderabbitai]

Context

During spawn phase, nukes are blocked via isSpawnImmunityActive() checks in nukeSpawn(), but structure spawning methods (landBasedStructureSpawn, portSpawn) lack equivalent guards.

NationExecution (which is active during spawn phase via activeDuringSpawnPhase() => true) calls structureBehavior.handleStructures(), which then calls player.canBuild() for structures. This allows structures to be queued for construction during spawn phase via the nation AI path.

Proposed Solution

Add isSpawnImmunityActive() checks to structure spawn methods to match nuke behavior:

• Either check inside canBuild() before calling canSpawnUnitType() for structure types
• Or add early-return checks to landBasedStructureSpawn() and portSpawn() (mirroring nukeSpawn())

References

• Related PR: Fix/Perf/Refactor: playerActions and buildableUnits, their callers and related types #3220
• Comment: Fix/Perf/Refactor: playerActions and buildableUnits, their callers and related types #3220 (comment)
• nukeSpawn blocks during spawn at line ~1194 in src/core/game/PlayerImpl.ts
• Structure spawns have no equivalent guard

[VariableVince]

@FloPinguin assigned to you, in case Rabbit is right about this. It was out of scope for my PR so i asked it to create this Issue. But there may well be checks that prevent this elsewhere in your Natiom logic, haven't checked that.

[FloPinguin]

Even though NationExecution is active during spawn phase, it never calls handleStructures() during that phase - it only handles player spawning. The structure building code path is unreachable during spawn phase, making the proposed fix unnecessary.

[VariableVince]

@CodeRabbit See the above comment by FloPinguin and save it in your learnings

[coderabbitai]

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@VariableVince, I've saved FloPinguin's clarification to my long-term memory. This will help prevent similar concerns in future reviews about structure spawning during the spawn phase.

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✏️ Learnings added

Learnt from: VariableVince
URL: https://github.com/openfrontio/OpenFrontIO/issues/3287

Timestamp: 2026-02-25T21:35:31.597Z
Learning: In NationExecution (src/core/execution/NationExecution.ts), even though it's active during spawn phase via activeDuringSpawnPhase() => true, it never calls handleStructures() during that phase - it only handles player spawning. The structure building code path is unreachable during spawn phase, making spawn immunity checks for structure spawning unnecessary.

Note: Learnings are effective only in the context of similar code segments. To apply general coding guidelines, please configure review instructions. You can manage existing learnings in the app.

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🧠 Learnings used

Learnt from: scottanderson
Repo: openfrontio/OpenFrontIO PR: 1851
File: src/core/execution/FakeHumanExecution.ts:433-441
Timestamp: 2025-08-18T03:11:18.617Z
Learning: In FakeHumanExecution.maybeSpawnStructure, the perceived cost multiplier (owned + 1, capped at 5) applied to structure costs is intentional. This makes AI nations build fewer structures than players could, saving gold for bombs/nukes rather than maxing out structures.

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Learnt from: sambokai
Repo: openfrontio/OpenFrontIO PR: 2225
File: src/core/execution/FakeHumanExecution.ts:770-795
Timestamp: 2025-10-27T09:47:26.395Z
Learning: In src/core/execution/FakeHumanExecution.ts, the selectSteamrollStopTarget() method intentionally allows MIRV targeting when secondHighest city count is 0 (e.g., nuclear endgame scenarios where structures are destroyed). This is valid game design—"if you can afford it, you're good to go"—and should not be flagged as requiring a guard condition.