Implement r2modmanPlus-style BepInEx install rules with marker-based cache validation

[Claude]

Fixes "sometimes missing files" issue caused by stale extraction cache and adds support for all BepInEx route types (patchers, core, monomod) with proper nested folder handling.

Changes

Cache correctness (downloader.ts)

• Marker file _extracted.ok gates cache hits—both extractPath and marker must exist
• Cleans stale/partial extractions before re-extracting
• Removes marker on any extraction failure
• Logs cache hit/miss reasons (extractPath exists, marker exists, ignoreCache)

Nested folder support (mod-installer.ts)

• resolveEffectiveRoot() handles SomeFolder/BepInEx/... zip layouts common on Thunderstore
• Checks top level first, then immediate children (deterministic sort)
• Logs resolution decision and chosen path

BepInEx routes (mod-installer.ts)

• Installs all routes when present: plugins/, patchers/, core/, monomod/ (namespaced), config/ (merged)
• Idempotent: removes destination folders before copying
• Fallback preserved for non-BepInEx mods
• Logs which routes existed and were installed with file counts

Uninstall (mod-installer.ts)

• Removes BepInEx/{plugins,patchers,core,monomod}/<modId>
• Config files untouched (shared between mods)
• Logs removed paths and counts

Example

Before: A mod with BepInEx/patchers/ would only install plugins/, silently dropping patchers. If extraction was interrupted, partial cache would be reused.

After: All routes install correctly. Interrupted extractions trigger clean re-extraction.

// resolveEffectiveRoot handles this layout:
// ModName/
//   SomeFolder/
//     BepInEx/
//       plugins/
//       patchers/  ← now installed
//       core/      ← now installed

const effectiveRoot = await resolveEffectiveRoot(extractedModPath)
// Returns: /cache/ModName/SomeFolder (not /cache/ModName)

Original prompt

Goal

• Make “generic BepInEx rules for all games” behave like r2modmanPlus for the common BepInEx layout, specifically fixing missed installs of BepInEx/core and BepInEx/patchers and eliminating “sometimes” caused by stale/partial extraction cache.
  Non-goals (for this first pass)
• Implement the full r2modmanPlus rule engine (extension-based routing + best-fit rule matching across arbitrary folder layouts).
• Track installed files in _state/*.yml like r2modmanPlus.
• Handle game-specific extra routes (Assets, Sideloader, etc.). This is “generic BepInEx only”.

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Specifications
A) Extraction cache correctness (marker-based cache hit)
Applies to: electron/downloads/downloader.ts
Marker file

• Name: _extracted.ok
• Location: /_extracted.ok
• Contents: arbitrary (e.g. ok\n)
• Semantics: “This extraction completed successfully.”
  Cache hit rule (new)
• A cache hit is ONLY when:
  1. extractPath exists AND
  2. /_extracted.ok exists
     If either is missing, treat cache as invalid and re-extract.
     Re-extract rule (new)
• Before extracting (when not a valid cache hit):
  • If extractPath exists, delete it recursively.
  • Recreate it.
  • Extract zip into it.
  • After extraction succeeds, write _extracted.ok.
    Failure handling
• _extracted.ok must never exist for a failed or cancelled extraction.
• If extraction fails, leave no marker; optionally delete extractPath to avoid partial junk (recommended).
  Rationale
• Today you treat “folder exists” as valid cache; that’s vulnerable to partial extraction from a crash/cancel and is a primary source of “sometimes missing files”.

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B) Effective root resolution (nested top-level folder zips)
Applies to: electron/profiles/mod-installer.ts
Definition

• effectiveRoot is the directory that should be treated as the “package root” for installation.
  Algorithm

1. If /BepInEx exists => effectiveRoot = extractedModPath
2. Else list immediate children of extractedModPath:
   • Consider only directories
   • Sort deterministically (e.g. name.localeCompare)
   • If any child has /BepInEx => effectiveRoot = child (first match)
3. Else effectiveRoot = extractedModPath
   Case handling

• On Windows, treat BepInEx folder existence case-insensitively (practically pathExists(join(root, "BepInEx")) is enough if the FS is case-insensitive; no extra work unless you want to be explicit).
  Rationale
• A common Thunderstore zip layout is SomeFolder/BepInEx/.... Without this, your installer falls back and misplaces everything under plugins.

---

C) Generic BepInEx route handling (r2modmanPlus-style routes)
Applies to: electron/profiles/mod-installer.ts
When effectiveRoot/BepInEx exists
Install these routes if present in the extracted content:

1. Plugins (namespaced / “subdir”)

• Source: /BepInEx/plugins
• Destination: /BepInEx/plugins//...
• Behavior: copy entire source folder contents into folder (preserve structure under plugins/)

2. Patchers (namespaced / “subdir”)

• Source: /BepInEx/patchers
• Destination: /BepInEx/patchers//...

3. Core (namespaced / “subdir”) — per your decision

• Source: /BepInEx/core
• Destination: /BepInEx/core//...

4. Monomod (namespaced / “subdir”)

• Source: /BepInEx/monomod
• Destination: /BepInEx/monomod//...

5. Config (merged / “none”)

• Source: /BepInEx/config
• Destination: /BepInEx/config/...
• Behavior: merge copy (overwrite same-name files), not namespaced.
  Idempotency / reinstall semantics
• Before copying any namespaced routes, delete the destination folders (if they exist):
  • BepInEx/plugins/
  • BepInEx/patchers/
  • BepInEx/core/
  • BepInEx/monomod/
• Do NOT delete or “clean” BepInEx/config during mod reinstall/uninstall.
  When effectiveRoot/BepInEx does NOT exist
• Fallback behavior (keep close to current):
  • Copy everything from effectiveRoot into /BepInEx/plugins//...
    Counting / return values
• Today filesCopied is inconsistent (config increments per Dirent, not per file recursively).
• Spec option:
  • Either keep as “best-effort” count, or change copyDirectory to return the number of files it copied (recommended so the UI reflects reality).

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D) Uninstall logic must match install destinations
Applies to: electron/profiles/mod-installer.ts
Uninstall removes (new)

• /BepInEx/plugins/
• /BepInEx/patchers/
• /BepInEx/core/
• /BepInEx/monomod/
  Uninstall does NOT remove
• Anything under /BepInEx/config (shared/untracked like r2modmanPlus trackingMethod: none)

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Implementation Plan (step-by-step)

1. Update downloader cache semantics
   Files: electron/downloads/downloader.ts

• Add helper:
  • const markerPath = join(extractPath, "_extracted.ok")
• Replace cache hit check:
  • from: if (!ignoreCache &...