docs(glossary): phase 3 keep dt in main text only

[PLeVasseur]

Summary

• Remove glossary-side term definitions and keep canonical :dt: definitions in chapters.
• Add phase-3 guardrails for canonical target ownership.

Reference alignment

• Follow-up to glossary: generated-only glossary source with reproducibility checks rust-lang/fls#656 and Tshepang phased strategy (step 3/5).
• No glossary-entry/glossary-include directives are introduced.

Testing

• ./make.py --clear
• ./make.py --check-links
• uv run ./tools/glossary-migration-check.py --phase 3 --strict --report "$REPORT_ROOT/glossary-phase3-check.json"

[PLeVasseur]

Restack update: this branch was synchronized after PR6/PR7 remediation.

• Updated upstream lineage now includes:
  • fix(glossary): close alias coverage gaps in phase checks (c5db56a)
  • docs(glossary): align cast and casting definitions (be0f38f)
• Branch glossary-step-3-dt-main-only was merge-restacked from glossary-step-2-align-duplicate-text at 663a8a3.
• Please refresh diff context/review comments against the new base.