fix usage dashboard reconcile and responsiveness

[cosmiclasagnadev]

Make usage aggregation rebuild session data atomically and move dashboard persistence and view work off the hot path so navigation stays responsive while refreshing history.

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Note

Medium Risk
Touches core aggregation/reconciliation paths and changes when events trigger recompute, which could impact correctness of counts or freshness if edge cases were missed, but scope is limited to the usage dashboard plugin.

Overview
Usage aggregation is refactored to be atomic and less error-prone. putMsg/putTool/putSess now write into an explicit Agg instance (no global seen signatures), tool aggregation ignores non-terminal updates, and reconciliation rebuilds a session’s buckets into a fresh aggregate and then swaps them in to avoid double-counting/stacking.

Dashboard responsiveness is improved via caching and lifecycle cleanup. Adds bounded memoization for computed flats/rows/active-session counts keyed by store.rev, clears these caches on seed/reconcile, persists view prefs asynchronously on exit, and simplifies plugin event handling to just schedule reconciles; also resets state on plugin dispose.

Adds test coverage for aggregation and reconcile/seed failure behavior, updates README install instructions, and bumps version to 1.0.2.

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