Remove dormant user-location SSE subscription

[stef-k]

Summary

• remove the unreachable mobile SubscribeToUserAsync interface and implementation
• remove tests and current documentation owned only by the nonexistent mobile user-location endpoint
• retain shared Bearer and SSE Accept coverage through the active group subscription
• preserve all active group and visit SSE behavior

Closes #256

Product behavior

WayfarerMobile no longer exposes a dormant method targeting the nonexistent endpoint:

/api/mobile/sse/location-update/{userName}

Current production has no caller for that method. Active authenticated group and visit streams remain unchanged, including their event parsing, reconnection, cancellation, disposal, stale-client isolation, and visit polling fallback.

No replacement or redirect to Wayfarer's distinct generic web location stream is introduced.

Compatibility

• released mobile versions contained the method but had no production call site
• installed clients and backend-first deployment compatibility are unaffected
• Wayfarer backend code and endpoints are unchanged
• no authentication, Timeline synchronization, Trip, routing, persistence, schema, or migration changes

Validation

• SseClientTests: 30 passed
• group/SSE-management selection: 69 passed
• visit notification/fallback selection: 55 passed
• complete suite: 1,886 passed, 0 failed, 0 skipped
• Release test/Core build: passed, 9 retained unrelated warnings
• Android Release build: passed, 0 errors, 133 retained package/XAML warnings
• final dead-surface searches: no remaining method, route, or stale user-channel claim
• git diff --check 89d5404c...HEAD: passed
• no real service/provider contact

Code Guard

Code Guard reports SseClient.cs at 640 counted LOC, reduced from 657. The user explicitly approved a narrow exception for issue #256 at exact head db4462bb18e5b1666bed5a61fa03ae1a9b9abb9e.

The exception:

• applies only to this shrinking dead-surface-removal change;
• does not authorize future growth;
• creates no baseline, suppression, exclusion, grandfathering rule, or threshold change;
• does not waive other findings.

No other Code Guard FAIL or INCOMPLETE exists.

UI evidence

No screenshot is needed: no reachable UI behavior changes.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
WayfarerMobile.Core	87%	79%	1000	✔
Summary	87% (1381 / 1579)	79% (691 / 874)	1000	✔