Fix release-readiness findings and release 1.6.3

[wlaur]

Works through the findings in REVIEW.md in order of severity, then bumps the patch version to 1.6.3.

CRITICAL

• ReadTheDocs build fixed. nbsphinx executed the example notebooks in a separate Jupyter kernel that gets neither conf.py's sys.path insert nor an installed encomp (RTD uses --no-install-project), so every RTD build broke from PR Address release readiness review #13 onward and the 1.6.2 docs never published. Render committed notebook outputs on RTD (nbsphinx_execute="never" when READTHEDOCS is set); CI's sphinx-build -W (real plugin, executes notebooks) stays the correctness gate. Stale .readthedocs.yaml comment updated.

MAJOR

• docs/usage.md media-dimensionalities claim corrected. Only normal and currency are predefined; media dims (dry_air/fuel) are user-defined via define_dimensionality(), and water is a pint density unit that can't be used as a media tag.

MINOR

• docs-rtd CI job rebuilds the docs in an RTD-equivalent environment (no project install, READTHEDOCS set); publish now depends on it, so an RTD-only break fails the PR instead of silently landing.
• FrequencyUnits literal + __new__ overloads: Q(50, "Hz") now infers Frequency statically and Hz is in get_registered_units().
• README: documented the stray-.env import-failure footgun.
• Test-debt cleanup in test_units.py: assert the current redirect behavior instead of a disabled negative test; narrow broad pytest.raises(Exception) to DimensionalityTypeError.

NIT

• Reworded the "base class cannot be instantiated" note; @lru_cache on lib_version(); deleted the redundant Water.REPR_PROPERTIES; README changelog pointer (GitHub Releases) + in-wheel test-suite note.

Tests

• Meaningful public-API tests for previously-uncovered behavior (isinstance_types non-Quantity unions, convert_volume_mass Series-density validation, convert_gas_volume invalid condition, Dimensionality subclass errors, coolprop invalid assume_phase, Quantity.check() with a Dimensionality instance / raw pint quantity, Fluid/Water/HumidAir fixed-point-count validation, describe() unknown name, Frequency inference). conversion/gases/misc reach 100%.

Local verification

• pyright (strict): 0 errors. pyrefly check: 0 errors (42 suppressed). ruff check/format: clean.
• pytest: 694 passed.
• Docs build clean with -W both ways: RTD path (READTHEDOCS=True, notebooks not executed) and CI path (notebooks executed with the real plugin).

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