fix: compare serialized timestamps in seconds

[thepastaclaw]

Issue being fixed or feature implemented

The Dash-specific adaptation in dashpay#7459 compares serialized int64_t timestamps in seconds with native-resolution NodeClock::time_point values. The implicit common-duration conversion scales peer-controlled values to microseconds or nanoseconds and can overflow signed integers before the comparison.

What was done?

• Normalize adjusted time to whole seconds before comparing it with serialized spork and governance timestamps.
• Make the governance vote-rate boundary check safe at extreme clock values without subtracting potentially unrepresentable durations.
• Preserve existing wire and disk timestamp types and encodings.
• Add focused regression coverage for signed extreme spork timestamps and a validly signed governance vote with an extreme future timestamp.

How Has This Been Tested?

Tested on macOS/aarch64 with an undefined-behavior-sanitized build:

• CCACHE_DISABLE=1 make -C src -j15 test/test_dash
• UBSAN_OPTIONS=halt_on_error=1:print_stacktrace=1 ./src/test/test_dash --run_test=chrono_overflow_tests --log_level=message
• UBSAN_OPTIONS=halt_on_error=1:print_stacktrace=1 ./src/test/test_dash --run_test=chrono_overflow_tests/governance_vote_future_time_extreme --log_level=test_suite
• UBSAN_OPTIONS=halt_on_error=1:print_stacktrace=1 ./src/test/test_dash --run_test=chrono_overflow_tests,governance_vote_wire_tests,governance_validators_tests,coinjoin_queue_tests,ratecheck_tests --log_level=message
• clang-format --dry-run --Werror src/test/chrono_overflow_tests.cpp
• git diff --check f5e834c0f9011e523725684c543b055a7ea86621..HEAD

All tests and checks passed.

Breaking Changes

None.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

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[thepastaclaw]

Closing as superseded: the target branch was force-updated to 40e554c and now contains the serialized-timestamp overflow fixes and focused regression coverage directly. This child PR is therefore redundant and conflicted. Fresh CI/review continues on dashpay#7459 at that head.