Factor out topic discovery pattern from managers

[rolker]

Problem

Several managers in CAMP independently implement the same topic discovery pattern:

• AISManager::scanForSources()
• GridManager::scanForSources()
• MarkersManager::scanForSources()

Each one creates a QTimer (1s interval), calls node_->get_topic_names_and_types(), filters by message type string, and subscribes to new matches while tracking seen topics in a map.

PlatformManager currently uses a hardcoded topic name instead of this pattern, but could benefit from discovery too.

Proposal

Factor the scan-and-subscribe pattern into a shared utility (e.g., a base class or helper in camp_ros) that managers can use with just a message type filter and a subscription callback. This would:

• Eliminate duplicated boilerplate across 3+ managers
• Make PlatformManager resilient to topic namespace changes
• Provide a consistent pattern for future managers

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Authored-By: Claude Code Agent
Model: Claude Opus 4.6