test: raise meaningful coverage gate to 96%

[jhgaylor]

Summary

• add behavior-focused coverage for trusted inline HTML and image sanitization
• exercise guardrail failures for missing pages, invalid page responses, and malformed search-index JSON
• raise the deterministic coverage gate from 90% to 96%
• bump the package from 0.1.0 to 0.1.1 and document the release

Coverage moved from 94.74% (110 tests) to 96.93% (114 tests). I stopped there: the remaining misses are defensive MDEx/Lumis failure fallbacks and parser states that valid markdown input cannot produce without mocking private dependencies.

Checks

• 114 tests, 0 failures (mix test --cover)
• formatting clean
• compile with warnings as errors clean
• Credo strict clean
• unused dependency lock check clean
• Hex package build clean
• release guard clean; 0.1.1 is unpublished

Dialyzer is left to CI for the authoritative Linux result because parallel local runs contended while rebuilding Dialyxir's shared cold PLT.