Add explicit provider-native routing modes

[stef-k]

Summary

• Fresh online routing presents only backend-provided provider-native modes.
• No mode is preselected and no Transport Profile inference occurs.
• The exact selected mode flows through capability, route request, validation, publication, and retained provenance.
• Saved Segment and retained-route priority remains unchanged.
• Direct remains explicit and network-free.
• Chooser dismissal leaves navigation and dropped-pin state unchanged.
• Stale chooser completion cannot cancel or overwrite newer routing work.
• Older backends without additive modes produce bounded hosted-routing unavailability.
• Retained schema version 11 is additive and does not rewrite existing routes.
• Mapbox Directions remains unsupported.
• The backend repository was not modified.
• Documentation and changelog are updated.

Delivery step 2 of 3 for the related backend issue: stef-k/Wayfarer#538

Validation

• Complete Release suite: 1,978/1,978 passed.
• Release tests/Core build: success.
• Android Release build: success.
• Existing AndroidX NU1608 dependency warnings are classified as pre-existing.
• git diff --check: passed.
• No live provider contact.
• No iOS build because no suitable macOS/Xcode host was required.
• Screenshot: infrastructure-unavailable. The configured Android SDK path was absent; the established fallback SDK and AVD were diagnosed and retried once, but the emulator never appeared in ADB. No new emulator strategy was created.

Code Guard

• Overall: FAIL.
• Sole hard-cap finding: existing 1,190-line MainViewModel.cs.
• The exact branch change in that file is only the nullable return signature.
• The user explicitly authorized a one-time exception for that line.
• No configuration or broader exemption was added.
• No other FAIL or INCOMPLETE.

Notes

• Authorized one-line MainViewModel.cs LOC exception applies only to this delivery step.
• This PR does not merge, release, publish, or deploy anything.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
WayfarerMobile.Core	88%	80%	995	✔
Summary	88% (1398 / 1594)	80% (689 / 866)	995	✔