fix(runner): ensure atomic resume.cfg persistence and in-flight target drain on interrupt

[gcoinstash-cmd]

Description

Addresses intermittent resume.cfg corruption and incomplete target state persistence during SIGINT/SIGTERM interrupts.

Fixes #2345
/claim #2345

Key Changes

• Drain Synchronization (runner/runner.go): Synchronizes worker termination with target flushing on interrupt so in-flight requests finish before channel teardown.
• Contiguous Offset Tracking (runner/resume.go): Introduces sequential index completion mapping to ensure out-of-order responses do not skip unprocessed target pointers.
• Atomic File Serialization (runner/resume.go): Implements atomic temp-file write + fsync + rename semantics to prevent partial file writes upon sudden termination.
• Regression Test Coverage (runner/resume_test.go): Added atomic save, out-of-order completion, and multi-threaded interrupt/resume test cases.

Verification

• go vet ./...: Passed (0 warnings)
• go test -count=1 ./...: Passed (100% test pass rate)

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