fix(watchbound): preserve reserved-directory copies on Node 26

[VirgilMing]

Summary

Enabling directory-only-working-tree-watch on a build host running Node 26.10.0 fails in the production resource commit with ERR_FS_CP_EEXIST. The transaction reserves its package root with mkdir and then calls cpSync on that existing directory with force: false, errorOnExist: true. Node 26.10.0 now enforces the existing-directory refusal (release notes, Node #64124).

Copy each package entry into a missing child of the held reservation FD. Keep the atomic root reservation, descriptor/inode checks, no-overwrite options, commit checks, and quarantine rollback. Refuse pre-existing child entries before copying so older Node versions cannot merge an existing child directory.

Add or strengthen regression coverage for nested/hidden entries and empty subdirectories, empty and populated destination conflicts, existing file/directory/symlink children, reservation replacement before and between copies, and rollback after a production copy commits only part of a package.

This is an independent sibling of the Gentoo foundation PR #1564, based directly on main; neither PR depends on the other. The change stays in the community feature's build-time resource copier and adjacent tests. Gentoo feature support declarations, the official GUI runtime, and the Watchbound native engine are unchanged.

Validation

• Full feature suite on Node 24.19.0: 126 passed.
• Full feature suite on Node 26.10.0: 126 passed; unchanged main had 17 failing feature tests.
• Linux feature and ASAR framework suites: 35 passed.
• Signed stable 26.1002.52244 / amd64, with only directory-only-working-tree-watch enabled, built successfully on Node 26.10.0. Both Watchbound descriptors applied; the existing required core focus patch also applied.
• Before using the downloaded deb as build input, verified the pinned-key InRelease signature, the indexed Packages digest, and the complete package size/SHA-256. After building, verified the complete inventories and hashes of all three Watchbound packages (22 files), the unpacked x64 native binding, and absence of the ARM64 package.
• Node syntax and git diff --check: passed.

The local official-bundle build covers amd64. ARM64 runtime and the full deb/RPM/pacman/AppImage/Nix package matrix were not rerun locally for this copier-only change; upstream PR CI supplies the repository matrix. No system package merge or GUI replacement was performed.

Agent review

The complete origin/main...HEAD diff (61b1663164e3f141ba1a307cc61a1d4e3c8fb276 to b58ed033631de1ed486975b6164404734b296326) was independently reviewed read-only with gpt-6.1-sol at maximum reasoning effort. The result reports no actionable regressions relative to that merge base.

The reviewer also passed syntax checks, eight read-only feature tests on Node 26.10.0, and the diff check. The full filesystem-writing suites and official-bundle build listed above were performed separately in the writable local validation environment.

Checklist

• This pull request is ready for review and is no longer a draft.
• The change is focused, edits feature-owned source files rather than generated output, and contains no unrelated changes.
• Added regression tests and ran the local validation listed above.
• Reviewed the complete base-to-HEAD diff at maximum reasoning effort and addressed all actionable findings.
• Required upstream PR CI checks pass (pending after creation).

[ilysenko]

Reviewed the focused Node 26 reservation-copy fix and regression coverage. CI is green on the current head.