feat(anima): add prompt weighting support

[BMFreed]

Summary

Adds prompt weighting support to Anima text conditioning.

• Supports (text), (text:weight), nested groups, and escaped literal parentheses.
• Removes weighting markup before Qwen3 encoding.
• Maps weights onto T5-XXL tokens using tokenizer offset mappings.
• Passes the resulting weights through the existing Anima conditioning pipeline.

The implementation was written independently. ComfyUI was used only as a behavioral reference; no GPL-licensed source was copied or adapted.

Related Issues / Discussions

N/A

QA Instructions

• Verified that an unweighted prompt and the equivalent (text:1) prompt produce identical cleaned text, T5 token IDs, and weights.
• Verified that weighted text receives the requested T5 token weights.
• Verified that Qwen3 receives the cleaned prompt without weighting markup.
• Manually verified Anima generation in InvokeAI 6.13.8 using matching seed and generation settings.
• Ran the Anima test suite: 211 passed, 2 skipped.

Merge Plan

No special merge requirements.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated
• No redux slice changes
• No documentation changes required
• Updated Whats New copy (not a release PR)