Avoid oversized cycle detector allocation

[JSap0914]

Summary

• replace the fixed 1e6-entry call stack buffer in Service with a grow-on-demand runtime.Callers buffer
• add a regression test that fails when a cache-missing Service call allocates more than 1 MiB

Fixes #19.

Verification

• go test -run TestContainer_Service_AvoidsOversizedCycleDetectorAllocation -count=1 ./...
• gopls check izidic.go izidic_test.go
• git diff --check
• go test -count=1 ./...

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