feat: add owned semantic export API

[aaaaaandrew]

Summary

• add an opt-in semantic-export API for caller-supplied PHP documents, with owned declarations, occurrences, calls, byte ranges, and diagnostics
• resolve every document in one shared headless backend and provide deterministic batch and streaming output
• add an independent offline-stubs feature so offline consumers can build without triggering a stub download
• keep editor builds unchanged unless the export feature is enabled; the cold-start completion benchmark measured 1.811 ms on main and 1.766 ms with this patch

The generated-corpus export benchmark processed 24,000 documents / 146 MB of source in 1.50–1.56 seconds at the default cache window, with peak RSS holding between 476.7 and 478.3 MiB.

Closes #328

Test plan

• cargo test
• cargo clippy --features semantic-export --all-targets -- -D warnings
• cargo fmt -- --check
• cargo test --no-default-features --features semantic-export semantic_export::tests
• offline exporter tests with semantic-export,offline-stubs and network access disabled
• cargo build --release --features semantic-export,offline-stubs

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[aaaaaandrew]

CI update: the previous benchmark ran from a stale, conflicting base and compared against newer main results. I’ve rebased onto current main; the PR is mergeable and local exporter tests/clippy pass. The replacement CI run is awaiting maintainer approval. All non-benchmark jobs passed previously.

[AJenbo]

The benchmark often fails because of permission issues, but thanks for looking in to it.