Clarify that same_shape() compares abstract UTN #57 written units, not font-specific rendering

[satsragclaw]

Problem

same_shape() compares the abstract written-unit sequences produced by this project's UTN #57 / mongfontbuilder-derived shaping model. It does not invoke HarfBuzz with a specific font or compare rendered glyph IDs or outlines.

The current name and wording around "visual identity" can be interpreted as a guarantee that two strings render identically with every Mongolian font and platform. Real-world fonts may differ in FVS support, OpenType rules, or implementation details.

Proposed changes

• Clarify in the README and API docstring that same_shape() means equality under mongol-norm's abstract shaping model.
• Distinguish Mongolian-specific shaping equivalence from font-specific rendering equivalence.
• Document that applications requiring a particular font's final rendering should validate with HarfBuzz and that font.
• Consider adding a short "Model and limitations" section with an example of the distinction.

Acceptance criteria

• Public documentation no longer implies universal font-independent visual equivalence.
• The reference model (UTN #57 v4 plus mongfontbuilder-derived data/rules) is named explicitly.
• The behavior and API remain backward compatible.