docs: clarify evidence bundle observability

[DwarfM42]

Summary

• Reframes What you get from a bundle file inventory into the concrete observations and checks a user can make from saved evidence.
• Limits every public claim to behavior confirmed against the current runtime implementation, draft schema, and sample evidence.
• Retains the cross-platform qualification boundary, sampled-versus-exact distinction, typed omission contract, and separation between Probe measurement state and workload outcome.

Validation

• bash scripts/cargo-local.sh fmt --check
• bash scripts/cargo-local.sh test --all-targets --locked -- --nocapture
• bash scripts/cargo-local.sh check --all-targets --locked
• bash scripts/cargo-local.sh build --release --locked --bin perf-probe
• git diff --check
• README fragment-link validation and cross-platform sample-artifact assertions