0.15.2 hardening: xcnt truncation warning, renewal guard, CSV engine

[derrynknife]

Three small fixes for the 0.15.2 patch, closing #195, #206 and #207:

• xcnt_handler warns when right-censored observations carry a finite right-truncation time (Data validation: warn on right-censored observations with finite right truncation #195): the combination is contradictory (right truncation means the event was seen before tr; right censoring says it is after the censoring time) and can make truncation-adjusted likelihoods unbounded.
• RenewalModel.from_dict validates that the stored distribution name resolves to a genuine ParametricFitter (Serialisation hardening: RenewalModel.from_dict lacks the ParametricFitter guard on its distribution lookup #206), matching the guard in every other serialisation reader; the ARI branch already resolved through the restricted intensity map.
• The bundled dataset loaders drop engine="python" (Chore: drop engine="python" from the bundled-dataset read_csv calls #207): all 12 loaders parse identically with pandas' default C engine, now locked in by a loader test sweep.

Tests: warning fires exactly when it should (and not for inf-tr or uncensored rows), renewal reader rejects a real-but-wrong attribute name, all 12 loaders return non-empty frames. flake8/black/mypy clean.

Generated with Claude Code
https://claude.ai/code/session_01TRhKL2fJBiAAfNo9rihwts

---

Generated by Claude Code