refactor: extract GEXF export from the Apotheosis facade (closes #18)

[Dani-giron]

Summary

apotheosis.rs mixed index coordination, persistence and GEXF visualization export (about 85 of its 250 lines, including manual XML string patching). Any change to the export format touched the same file holding the critical insert/search logic.

Changes

• src/export/gexf.rs (new): draw(), add_attribute_schema() and save_gexf() moved unchanged, now a free function export::gexf::draw(&model, path) consuming the facade through its public API.
• src/controllers/apotheosis.rs: draw() removed. Two read-only accessors added: draw_model() (graph shape, one entry per layer) and record(index). Both are needed by export::gexf::draw(), which lives outside this impl and cannot reach hnsw/records directly now that they are private ([Enhancement] Make Apotheosis internal fields private to protect the synchrony invariant #14/fix: make Apotheosis internal fields private (closes #14) #15).
• src/lib.rs: pub mod export;.

Breaking change, declared: callers of model.draw(path) move to export::gexf::draw(&model, path). Verified nothing in this repo calls draw() besides its own definition.

Test plan

Test coverage for this refactor will be added as part of the dedicated test suite work (tests are being consolidated separately instead of per-PR).

• cargo test, cargo check pass locally.
• rustfmt --check on the three changed files (apotheosis.rs, export/gexf.rs, export/mod.rs) passes. (src/lib.rs was excluded from the direct rustfmt check since it cascades to the whole module tree including unrelated pre-existing files.)
• cargo clippy on the full crate could not be verified locally (unrelated toolchain issue on this machine); will be validated by CI on this PR.

Closes #18

[danielhuici]

This functions shouldn't be necessary anymore (fixed at GEFX library)

[Dani-giron]

Heads up: this PR will conflict with #34.

This PR deletes the draw()/save_gexf() block in src/controllers/apotheosis.rs (lines ~200-291 on the current apotheosis2 tip) to move it into src/export/gexf.rs. #34 fixes the .unwrap() panic and the discarded I/O error inside that exact same block (touches lines 207, 234, 266, 279), without moving it.

Whichever of the two merges second will get a real conflict there, not a false positive, one deletes the lines the other modifies. If #25 goes first, the .unwrap()/discarded-error fix from #34 needs to be reapplied in the new src/export/gexf.rs location instead. If #34 goes first, this PR's diff will need a rebase to move the already-fixed version.