fix: preserve existing export documents and assets

[rogerdigital]

Summary

Execute T1-T2 of the 1.0.0 release readiness plan: protect all outputs from silent overwrite and reproduce the confirmed attachment-corruption defect with a persistent fixture.

T1 — Persistent fixture and corruption regression

• New src/test-support/memory-vault.ts: an in-memory vault whose writes persist across runs, with real mock TFile instances, text/binary storage, strict create/modify semantics (rejects duplicate creation, wrong-kind modification, aliases binaries), and a metadata cache limited to the fixture's literal wiki links.
• New src/export/ExportIntegrity.test.ts end-to-end test using the real runner, collector, rewriter and writer. Before the fix it failed exactly as planned: after two sequential single-note exports with overwrite disabled, exports/assets/img.png contained [2] instead of [1].

T2 — Protect all outputs

• OutputWriter now takes the overwrite policy in its constructor (default true preserves existing callers). External writes use the exclusive wx flag when overwrite is disabled; vault writes reject an existing destination and only create — modify requires overwrite enabled. Conflicts raise an explicit Output already exists error.
• copyBinaryFile routes through writeBinary (single policy implementation) and throws Attachment source not found for missing sources instead of silently skipping.
• ExportRunner relocates a single-file export whenever anything occupies its output root (including an existing empty directory), so earlier exports keep their documents, attachments and links intact.
• Export reports get conflict-free path allocation against the complete planned output list, and are written by a dedicated create-only writer: reports never overwrite anything, including prior reports, even with overwrite enabled.
• 10-case regression matrix covering sequential exports, empty/occupied roots, occupied timestamp/suffix candidates, batch leaf sharing, export-report.md name collisions, late-appearing destinations, explicit overwrite, and missing attachments.

Also: eslint.config.mts disables obsidianmd/no-nodejs-modules for test files (tests run under vitest in Node and are never bundled).

Validation

• npx vitest run src/export/ExportIntegrity.test.ts src/export/OutputWriter.test.ts src/export/ExportRunner.test.ts src/export/ExportPlan.test.ts src/export/LinkRewriter.test.ts — 145/145
• npm test — 378/378 (baseline 360 + 18 new)
• npm run lint:obsidian-warnings — exit 0
• npm run build — exit 0