docs: quarantine legacy debug notes and update deprecation inventory

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Motivation

• Remove clutter from top-level docs by moving old engineering/debug pages into an explicit legacy archive so the active docs nav stays focused.
• Preserve backward compatibility by keeping lightweight stub pages at the original paths instead of deleting anything.
• Record the sweep in the project deprecation inventory so future cleanup work is tracked and discoverable.

Description

• Moved detailed debug pages into an archive folder at docs/legacy/internal_notes/ for traceability while removing them from the active docs surface.
• Replaced the original top-level files with short deprecated stubs at docs/viewer_debug_notes.md and docs/streaming_renderer.md so existing inbound links continue to resolve.
• Updated docs/project/deprecation_inventory.md to log the quarantine action, add an "Old-stuff quarantine sweep (2026-03-27)" note, and list next candidate files for similar treatment.
• Fixed an internal link in the archived streaming note to point to the canonical vase_volumes.md path; no runtime code was changed and no files were deleted.

Testing

• Ran mkdocs build --strict, which surfaced existing strict-mode warnings (including git-revision-date warnings for the archived notes) and therefore failed in strict mode.
• Ran mkdocs build successfully to confirm the documentation still renders after the moves and stub replacements.
• Verified the new archived files live at docs/legacy/internal_notes/viewer_debug_notes.md and docs/legacy/internal_notes/streaming_renderer.md and that the stub pages exist at their original locations.

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Codex Task