fix: add beta 6 grammar shape pressure

[tryskian]

Summary

• add grammar-led shape pressure to the runtime prompt contract for the failed Beta 6 why pulse
• keep the correction phrase-bank-free and fixed-prompt compatible
• record the correction in decisions, research, architecture, and handoff docs

Validation

• make lint-docs
• git diff --check
• make check
• make end-preflight

Notes

• no live API pulse rerun in this branch; the docs still hold the rate-limit / prepaid-credit pause boundary