DOCX: add native conversion timeout and stage-level telemetry

[developer0hye]

Summary

DOCX conversion needs built-in timeout controls and stage-level telemetry to handle heavy documents predictably.

Problem

Some documents require excessive conversion time and currently depend on external wrappers for process-level timeouts. The library/CLI should provide first-class timeout behavior and timing visibility.

Proposal

1. Add configurable timeout budget.

• library: ConversionOptions::timeout
• CLI: --timeout-seconds <n>
• return explicit timeout error type

2. Add stage timing instrumentation.

• parse time
• IR -> Typst generation time
• Typst compile/export time

3. Improve timeout UX.

• clear error message stating which stage exceeded budget
• deterministic non-zero exit code in CLI

4. Add tests.

• unit tests for timeout option validation
• integration tests for timeout error mapping and exit behavior

Code Areas

• crates/office2pdf/src/config.rs
• crates/office2pdf/src/lib.rs
• crates/office2pdf/src/error.rs
• crates/office2pdf-cli/src/main.rs

Acceptance Criteria

• CLI can enforce conversion timeout without external shell wrappers.
• Timeout failures are distinguishable from parse/render failures.
• Stage timing is available in logs (debug mode) for profiling.
• Default behavior remains backward-compatible when timeout is not set.

Non-Goals

• No XLSX scope in this issue.