Add allocator observability and sustained-load validation

[antono3]

Summary

• add constant-time lifetime counters and high-water marks alongside current allocator statistics
• add an opt-in bounded, chronological lifecycle event trace with overwrite accounting and resettable measurement windows
• add deterministic 30,000-operation CPU planner validation and a separate 1,536-allocation real Vulkan stress workload
• document the policy/planning/ownership/observability layers and preserve the existing external synchronization contract
• prepare the additive API as v2.5.0

Validation

• released V 0.5.2 (7647ce1): v fmt -verify .
• released V 0.5.2: explicit v vet source list
• released V 0.5.2: 7/7 test files pass
• released V 0.5.2 + clang: 7/7 test files pass under ASan and UBSan
• v run setup.vsh --check
• lavapipe introductory allocation/budget/upload-ring example
• lavapipe sustained workload: 1,536 allocations, mapped writes and flushes, fragmenting refill cycles, 1,533 block reuses, and final coalescing
• antono2.memory v1.4.0: 8/8 test files pass, including multithreaded synchronized-allocator churn