Rename legacy s_-prefixed private fields to camelCase and enforce the rule

Problem

.editorconfig:73-85 mandates camelCase private fields (no _/m_/g_/s_ prefix, per CSharpGuidelines AV1702/AV1705) but keeps severity = suggestion because ~35 legacy s_-prefixed identifiers across ~34 files still violate it (e.g. Telemetry.s_confirmedVersion). The convention is documented but not compiler-enforced, so new violations slip in. Surfaced in #451 review.

Outcome

No prefixed private fields remain; the naming rule is warning, so new violations fail the build.

Acceptance criteria

• All s_/_/m_/g_-prefixed private fields in src/**/*.cs renamed to camelCase
• dotnet_naming_rule.private_fields_should_be_camel_case.severity bumped suggestion → warning (.editorconfig:78)
• Build green with no new naming warnings

Notes

Mechanical rename; exclude generated code. Low priority.