docs: align IA/ROUTES navigation behavior with shell primitives

[kabeier]

Motivation

• Keep documentation accurate with the actual shell primitive wiring so authors and engineers know the single source of truth for navigation surfaces and item wiring.
• Clarify the difference between top-nav utility responsibilities and primary destination navigation to avoid UI/UX implementation confusion.

Description

• Updated docs/IA_MAP.md to reflect current sidebar vs mobile tab behavior with explicit breakpoint semantics and the canonical sidebar/mobile route sets.
• Updated docs/ROUTES.md to match the IA language so both docs describe the same navigation model and behavior.
• Strengthened the canonical-source note to point to src/components/ui/primitives/shell/primitives.tsx for shell surface/item wiring and noted src/lib/view-models/routes.ts as the source for route metadata used by mobile tab generation.
• No runtime code changes were made; this is a documentation-only update to docs/IA_MAP.md and docs/ROUTES.md.

Testing

• Pre-commit formatting hooks (Prettier) ran successfully on the changed markdown files.
• Repo verification run in this workspace shows npm run test:run passed and npm run typecheck, npm run lint, and npm run format:check all passed.
• npm run build failed due to an environment network issue downloading the Next.js SWC binary, and npm run a11y:smoke could not complete because the build artifact was missing; docs:route-status-gate was skipped for the same environment limitation.

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Codex Task