Harden payload buffer bounds handling

[dallison]

Summary

• Validate payload buffer offsets and typed access extents before forming pointers.
• Harden string readers/writers, vector growth, and bitmap allocator initialization so allocation failures preserve or roll back state.
• Add regression coverage for forged offsets, string bounds, vector allocation failures, and bitmap allocator cleanup paths.

Test plan

• bazelisk test --lockfile_mode=off //toolbelt:payload_buffer_test