feat: Add Tantivy FON search MVP

[Fuyeors]

Summary

• Add an Axum search API backed by Tantivy 0.26.1 and a Jieba-based Chinese tokenizer.
• Parse the current fon-parser main-branch AST for mapping, sitemap, and content FON resources.
• Materialize sitemap bindings into concrete FRL URLs, apply title fallbacks, keep image values as remote URL pointers, and treat content as plain text/Markdown.
• Add repository-level FON mock data and a dependency-free CSR search console with rebuild, pagination, and infinite-scroll loading.

Verification

• cargo fmt --all -- --check
• cargo test --workspace — 5 tests passed
• cargo clippy --workspace --all-targets -- -D warnings
• cargo build --workspace --release
• Browser smoke test: rebuilt 5 documents from 3 mappings and returned 2 results for 搜索引擎.

This is an MVP for local/mock ingestion. Remote fetching, robots.txt handling, conditional HTTP requests, persistent crawl state, and production multi-tenant controls remain follow-up work.