chore: verify cross-file consistency and sync ToC

fix: update code examples and temporal references

feat: add missing conceptual coverage for 2026

fix: update model references to current generation

fix: update tool references and ecosystem state

fix: update hardware specs to current generation

chore: refresh ecosystem statistics and pricing

fix: correct factual errors across course

fix: update benchmark scores to early 2026 verified data

fix: update model lineups to March 2026 state

chore: sync Table of Contents with content updates

fix: cross-file consistency, naming, and cross-references

update: expand thin sections, add practical decision guidance

fix: practical projects code quality and API alignment

update: voice and multimodal accuracy, practical limitations