docs: map mobile navigation and feature boundaries

[Jaydearcadian]

Summary

• replaces the existing navigation map with a canonical contributor-facing route and feature-boundary reference
• documents the actual Expo Router structure, including the root <Slot />, auth stack, tab navigator, and root-level routes
• maps screen ownership, Zustand stores, persistence, shared components, cross-feature dependencies, and global error handling
• calls out high-risk boundaries around secret-key access, deep links, payment signing, contacts duplication, vault configuration, diagnostics, and polyfill order
• adds the map to the README documentation index

Why

The previous map contained machine-local file:///... links, described the root as a stack even though it renders through <Slot />, omitted the wallet-creation-success route, and mixed current behavior with aspirational boundary rules.

This update separates current implementation details from recommended dependency direction and explicitly documents overlapping ownership where it exists today.

Validation

• confirmed no machine-local links remain
• confirmed README links to the canonical map
• confirmed root <Slot /> behavior is documented
• confirmed wallet creation, import, tabs, send/signing, receive/scan, contacts, transactions, vault, settings, diagnostics, and global errors are covered
• ran git diff --check

Documentation-only change; no application behavior was modified.

Closes #370

[El-swaggerito]

\nThis PR is currently a draft.\n\nThe auto-merge automation will not merge draft pull requests.