docs(intern-23): error handling trace + test proof (#361)

[CodingMinh]

Summary

• intern-23 / Closes [intern-23] Error handling deep-dive: recover gracefully từ format mismatch & corrupt data #361 — trace ConvertError, PDF fallback chain, best-effort vs fail-fast.
• Proof: bench/INTERN23_ERROR_TRACE.md — 6 one-detailed cases with raw JSON (OK, corrupt PDF, mismatch, malformed DOCX, partial PDF).
• No converter code changes; no new error types.

Test cases (evidence in trace file)

Case	Exit	Result
OK txt / OK PDF	0	full_success
Corrupt PDF	1	failed
Mismatch .txt→.pdf	1	failed
Malformed DOCX	1	failed (fail-fast)
Partial PDF --no-pdf-ocr	0	partial_success + warning

Key code paths

• crates/core/src/lib.rs — ConvertError
• crates/core/src/conv/pdf/mod.rs — inspector → PDFium → pdf-extract
• crates/core/src/conv/docx.rs — fail-fast, no fallback

Closes #361

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