ROX-35259: avoid proto timestamp allocs in IsStartupProcessView

[ebensh]

Description

IsStartupProcessView converts two *time.Time values to *timestamppb.Timestamp
(via protocompat.ConvertTimeToTimestampOrNil) only to pass them to protoutils.Sub,
which just reads .GetSeconds() and .GetNanos(). This allocates two proto structs
per process indicator row — ~191M heap objects per reprocessing cycle in production profiles.

Fix: subtract time.Time values directly. The nil-SignalTime behavior is preserved
and made explicit (returns true, matching the existing TODO comment about ROX-31107).

User-facing documentation

• CHANGELOG.md is updated OR update is not needed
• documentation PR is created and is linked above OR is not needed

Testing and quality

• the change is production ready: the change is GA, or otherwise the functionality is gated by a feature flag
• CI results are inspected

Automated testing

• added unit tests
• added e2e tests
• added regression tests
• added compatibility tests
• modified existing tests

How I validated my change

• Existing unit tests in central/processbaseline/evaluator pass
• Behavioral parity verified: nil-SignalTime still returns true (startup process), matching the prior protoutils.Sub behavior which returns 0 for nil inputs
• Identified via pprof alloc_objects profile of Central under steady-state load (1.35h capture)

🤖 Generated with Claude Code

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