feat(morphology): reserve explicit ASL realization hooks

[AmitMY]

Summary

Add experimental ASL hooks in signwriting.morphology.ase:

• pluralize(fsw) has a TODO and returns the input FSW unchanged for now.
• repeat_action(fsw) and sustain_action(fsw) remain unimplemented and raise NotImplementedError. They reserve repetitive and sustained action-aspect hooks, not past-tense conjugation or automatic English suffix mapping.

No hook currently produces modified SignWriting. Grammar selection belongs upstream; validated sign-specific realization can be implemented here later.

Part of https://github.com/sign/synthetic-sw-mt-gen/pull/1. No automatic dispatch or release is included.

Validation

• 139 tests passed with pytest signwriting --ignore=signwriting/mouthing; optional mouthing dependencies are not installed locally.
• Pylint passed for the morphology package.
• Wheel packaging was verified when the package was introduced.
• Regression tests verify identity behavior for pluralize and explicit failures for the two aspect hooks.

[AmitMY]

Closing as requested: the morphology placeholder hooks are not the direction we want to pursue.