sdk: make FTS snippet highlighting unambiguous for literal HTML

[willwashburn]

Context

Follow-up from #406. The SDK search verb returns snippets whose FTS5 match
boundaries use literal <b> / </b> markers. When stored content itself
contains those tags, presenters cannot distinguish match markup from
user-authored content: stripping or translating every tag can silently alter
content or falsely highlight an unmatched phrase.

Scope

• Preserve a stable, machine-readable search result contract.
• Represent match boundaries unambiguously even when content includes literal
  HTML-like tags.
• Update Rust SDK tests and downstream presenters to prove literal <b>
  content is preserved and only actual matches are highlighted.

Acceptance

• A content record containing literal <b>already tagged</b> round-trips
  without deletion or false match highlighting.
• JSON and human presenters have documented, deterministic match-boundary
  behavior.
• Existing phrase, boolean, prefix, and session-scoped FTS searches remain
  unchanged.