fix(p2p): stop peer tracker gc ticker

[loglapa]

Closes #223

Summary

• stop the peer tracker garbage-collection ticker when gc exits
• release ticker resources when the tracker context is canceled

Testing

• go test ./...
• go build ./...
• go vet ./...
• golangci-lint run --timeout 10m
• go mod tidy -diff
• git diff --check

[mergify]

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• Queue this pull request

[loglapa]

The only failing check is the previously observed store.TestStore failure. A clean local rerun of go test ./store -count=1 passes, indicating a flaky failure unrelated to this peer-tracker change. I attempted to rerun the failed GitHub Actions job, but GitHub restricts reruns to repository admins; a maintainer needs to rerun run 32359311645.

[loglapa]

@Mergifyio queue

[mergify]

queue

☑️ Command disallowed due to command restrictions in the Mergify configuration.

Details

• sender-permission >= write

[loglapa]

Fresh CI on the rebased head confirms the prior store.TestStore failure is gone. The new failure is instead in upstream TestExchangeServer_partialRangeNotExpanded, added by #406 on main: its partial-range fixture has a nil underlying store and panics in Tail(). The already-approved #409 is the focused fix for exactly that fixture. This peer-tracker patch does not touch range handling; once #409 lands, rebasing/rerunning #407 should clear this blocker. I am keeping the changes separate to avoid pulling an unrelated test fix into this PR.