fix: preserve temperature meaning and release 1.9.2

[wlaur]

Summary

• reject persisted TemperatureDifference columns when a default absolute K or degR schema would silently reinterpret them
• retain unit(..., asdim=...) as the explicit, value-preserving reinterpretation boundary
• document the absolute/difference meaning of offset and multiplicative temperature units, including the inherent ambiguity of persisted K/degR metadata
• add regression coverage for default rejection, ordinary absolute conversion, explicit reinterpretation, and schema disambiguation of persisted K
• bump pyproject.toml and uv.lock to 1.9.2

The published v1.9.1 release notes were also corrected in place: INCOMP::MEG[0.5] / INCOMP::MEG-50% replace the invalid INCOMP::MEG[50%] example, and the compatibility note now names the Series dtype, mixed-container, and floor-division behavior changes.

Temperature contract

• degC / degF: absolute Temperature
• delta_degC / delta_degF: TemperatureDifference
• K / degR: usable for either, defaulting to absolute Temperature
• ambiguous persisted K / degR: interpreted by the QuantityFrame declaration
• asdim=: explicit opt-in to semantic reinterpretation

Local validation

• uv lock --check
• ruff check / ruff format --check
• pyright strict, pyrefly, ty: zero errors
• full default suite: 1050 passed, 2 expected oracle skips
• Sphinx HTML build with warnings as errors
• exact CI Rust fmt, clippy -D warnings, and no-default-features tests: pass