fix(export): preserve concurrent outputs during publication

[johnny4young]

Summary

• Share the non-replacing filesystem publisher between GUI batch exports and CLI --no-overwrite image, sidecar, manifest, and skipped-report writes. Enforce exclusivity when each output is published, rather than relying on preflight.
• Preserve a concurrent producer's replacement when the exclusive-create fallback fails. Surface the error without unlinking the public destination; on unsupported-rename volumes an incomplete file can remain.
• --no-overwrite preflight now checks image, sidecar, and batch report targets without following symlinks, so a dangling symlink is reported as an existing output before rendering instead of failing late with a generic write error.
• Remove duplicate sidecar error logging; the shared writer logs the underlying error once and maps it to writeFailed.
• Add real-filesystem failure ownership, late image/sidecar collision, late full-batch report collision, dangling symlink (publication and preflight), and default overwrite regressions.
• Document per-file publication and partial-run behavior; correct the pagination example; add a changelog entry.

Validation

• Final head 8084a05f0c25f965fbecf188ca1dc0b581bda715: CI run passed all jobs (Linux static checks, Sequoia 15 and Tahoe 26 native build/test lanes, both UI lanes, GitGuardian).
• Locally: every Linux-runnable make lint sub-check and make changelog-check passed; swift-format lint --strict and swiftc -parse passed on the changed Swift files.

Scope and limits

• Publication is per-file, not whole-bundle rollback. Unsupported-rename fallback may leave partial output after an I/O failure.
• Follow-ups not in this change: a late collision is reported as writeFailed rather than outputExists (same exit code); a late skipped-report failure stops before the manifest is written; a batch input whose image published but whose sidecar collided is reported as skipped without a manifest entry; the fallback re-stages the payload on every write to volumes without exclusive rename.

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