Define explicit thrust profile semantics

[SuperdoerTrav]

Separate samples from segments and make index/time bounds explicit, rejecting ambiguous profiles with focused API tests. This is a design/API review branch, not part of the bug-fix series.

Summary

Repository Policy Checklist

• If any part of this change was produced with an automated agent or LLM assistance, AGENTS.md was followed and this PR states what was verified by running it.
• Changes preserve the existing file layout (ssapy_toolkit/, tests/, demos/, docs/, scripts/, .github/).
• Package-code changes include tests under tests/, or this PR only changes docs/CI/metadata.
• New user-facing workflows include a runnable demo under demos/, or no demo is needed because:
• No generated outputs, downloaded data, images, notebooks with embedded outputs, binary media, or large artifacts are committed.
• Persistent data needed by this work is stored in SSAPy-Data or documented as an external input.

Validation

• pytest tests
• python -m ssapy_toolkit.run_all_demos --no-open when demos or user-facing workflows changed
• python scripts/check_repository_policy.py
• No existing test, tolerance, workflow, or policy check was weakened to make this pass.
• Every number quoted in this PR came from a command I ran on this change.