Add scale-gated unary minus to temperature (enables negative literals)

[cleishm]

Problem

Creating a negative absolute-temperature literal, e.g. -123_dc, failed to compile.

In C++ a user-defined literal never receives the sign, so -123_dc parses as -(123_dc). 123_dc produces a decicelsius (a temperature), and temperature defined ++/--/+=/-= but no unary operator- — so negating the result was ill-formed. This affected every absolute-temperature literal (_c, _dc, _mc, _f, …); delta literals were unaffected since delta already has unary minus.

Fix

Add a constexpr unary operator- to temperature, gated on a new _is_relative_scale trait (Scale::offset != 0):

• Celsius / Fahrenheit — zero sits above absolute zero → negation is meaningful → available.
• Kelvin — zero is absolute zero → a negated temperature is always below absolute zero → compile error.

This is a purely type-level gate with zero runtime cost. It intentionally does not enforce value-level physicality (e.g. -300_c is still constructible), consistent with the rest of the library, which does not enforce an absolute-zero floor anywhere (kelvin{-5}, 50_k - 100_k, etc. already compile).

Behavior

Expression	Result
-123_dc	decicelsius(-123) = −12.3 °C ✅
-40_f	fahrenheit(-40) ✅
-25_k	compile error

Tests

New negation section in thermo_test.cpp:

• A has_unary_minus concept asserting availability on celsius/decicelsius/fahrenheit and unavailability on kelvin/decikelvin/millikelvin (the compile-time "it fails" check).
• static_asserts + runtime TEST_CASE for values (-123_dc == -123, -(-5_c) == 5_c, …).

Full suite: 157 assertions, 30 test cases, all passing.

Version

Minor bump 2.0.0 → 2.1.0 (backward-compatible feature) across all four version files: CMakeLists.txt, idf_component.yml, docs/Doxyfile, vcpkg-port/vcpkg.json. The vcpkg port SHA512 is left untouched (set post-release, direct to main).