Fix panic on server atomic save edits on MacOS

[bep]

An atomic save (write temp file, rename into place) unlinks the inode the
watcher holds, so kqueue reports Remove for a file that's still on disk.
That took the delete branch and wiped the entire taxonomy subtree; the
following assemble then panicked in createMissingTaxonomies, where the
shifting tree.Get hit a not yet assembled *pageMetaSource.

Treat Remove of a path that still exists as an update, and use the
non-shifting GetRaw when checking for the auto created taxonomy node.

Fixes #15130

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

[gemini-code-assist]

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[Copilot]

Pull request overview

Fixes a hugo server rebuild panic on macOS/kqueue triggered by atomic-save editor behavior (temp write + rename-over), ensuring file watcher events don’t incorrectly delete taxonomy subtrees and that taxonomy creation checks don’t shift unassembled nodes.

Changes:

• Treat fsnotify.Remove/Rename as an update when the path still exists (atomic-save behavior on macOS/kqueue).
• Avoid shifting during rebuild taxonomy checks by using GetRaw, and add integration coverage for atomic-save rebuild scenarios.
• Refactor content-node helpers (seq conversions / auto-node detection) and move contentWeight logic into pageMetaSource.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
hugolib/site.go	Reclassifies Remove/Rename events as “removed” only when Stat fails, preventing false deletes on atomic saves.
hugolib/rebuild_test.go	Adds rebuild integration tests covering atomic-save edits for section bundles and content-backed taxonomy roots.
hugolib/page.go	Delegates pageState.contentWeight() to pageMetaSource.
hugolib/page__meta.go	Introduces pageMetaSource.contentWeight() to centralize content weight logic.
hugolib/integrationtest_builder.go	Adds test harness support for simulating atomic-save watcher events.
hugolib/content_map_page_contentnodeshifter.go	Switches helper calls to the helperContentNode receiver-based APIs.
hugolib/content_map_page_contentnode.go	Adds helper methods for seq conversion and auto-node detection, and wires pageMetaSource into weight provider interface.
hugolib/content_map_page_assembler.go	Uses GetRaw + auto-node detection to avoid shifting unassembled nodes during rebuild taxonomy creation.

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