Add positioned written-unit normalization API

[satsragclaw]

Summary

• add MongolianShaper.normalize_positioned_written_units() for ordered built-in dict records with exact unit and position fields
• validate flat slot positions (isol, init, medi, fina, control) against explicit Mvs, Nirugu, and Zwj context
• snapshot accepted sequences once so stateful inputs cannot change after validation
• reuse normalize_written_units() for canonical selection and exact reshape validation
• reject missing structural context instead of inserting ZWJ or returning partial output

This change does not add a positioned-record CLI or automatic control reconstruction.

Validation

• 203/203 tests pass on Python 3.11
• 203/203 tests pass on Python 3.6.8
• 177/177 core-hud cases pass
• 3507/3507 eac-hud cases pass, excluding the existing 5 UTN xfails
• 2237/2237 prefix-stability pairs pass
• 1993/1993 shape groups converge
• normalization table and compatibility goldens are fresh
• wheel and sdist build successfully
• installed-wheel positioned API smoke test passes
• independent adversarial review found no security or logic blockers