refactor(peer): centralize game root path policy

Game scanning, manifest validation, install recovery, migration, and download
cleanup each carried their own spellings and case rules for reserved entries.
Those copies had already diverged, which made it possible for one subsystem to
accept or expose a path that another treated as application-owned state.

Introduce one game_paths module for the canonical names and conservative
portable comparison policy. Keep context-specific predicates for manifest and
scanner protection versus cancellation preservation: cancellation still sweeps
its own version transaction scratch files, while install and migration state
survive. Reuse the constants for all production path construction sites.

Test Plan:
- `just test` -- passed (175 lanspread-peer tests and full workspace)
- `just clippy` -- passed
- `just fmt` -- Rust, TOML, and Prettier completed; the recipe remains blocked
  by 39 pre-existing rumdl issues in five unrelated Markdown files
- `git diff --cached --check` -- passed

crates/lanspread-peer/src/download/manifest.rs

@@ -7,6 +7,8 @@ use std::{
 use eyre::WrapErr;
 use lanspread_db::db::{GameCatalog, GameFileDescription};
 
+use crate::game_paths::{VERSION_INI, is_download_protected_root_name, portable_name_key};
+
 /// A remote manifest may describe at most this many filesystem entries.
 pub(crate) const MAX_DOWNLOAD_MANIFEST_ENTRIES: usize = 100_000;
 /// A single remotely described file may be at most one tebibyte.
@@ -21,8 +23,6 @@ pub(crate) const MAX_DOWNLOAD_COMPONENT_BYTES: usize = 255;
 pub(crate) const MAX_DOWNLOAD_RELATIVE_PATH_BYTES: usize = 900;
 const MAX_DOWNLOAD_DESTINATION_UNITS: usize = 1_000;
 
-const VERSION_INI: &str = "version.ini";
-
 /// One entry whose path and shape were validated as part of a complete manifest.
 #[derive(Clone, Debug)]
 pub(crate) struct ValidatedDownloadEntry {
@@ -259,7 +259,7 @@ impl<'a> ProtocolV7ManifestBuilder<'a> {
         if windows_alias_component(root_component)? == self.game_alias {
             eyre::bail!("download path contains a doubled game prefix: {display_path}");
         }
-        if is_protected_root_component(root_component) {
+        if is_download_protected_root_name(root_component) {
             eyre::bail!("download path targets install or recovery state: {display_path}");
         }
         Ok(())
@@ -337,7 +337,7 @@ fn validate_game_id(game_id: &str) -> eyre::Result<()> {
         eyre::bail!("catalog game ID must be one path component: {game_id}");
     }
     validate_component(game_id)?;
-    if is_protected_root_component(game_id) {
+    if is_download_protected_root_name(game_id) {
         eyre::bail!("catalog game ID is reserved for application state: {game_id}");
     }
     Ok(())
@@ -447,7 +447,7 @@ fn validate_component(component: &str) -> eyre::Result<()> {
 
 fn windows_alias_component(component: &str) -> eyre::Result<String> {
     validate_component(component)?;
-    Ok(component.to_uppercase())
+    Ok(portable_name_key(component))
 }
 
 fn is_windows_device_name(stem: &str) -> bool {
@@ -472,25 +472,6 @@ fn looks_like_dos_short_name(component: &str) -> bool {
     })
 }
 
-fn is_protected_root_component(component: &str) -> bool {
-    let alias = component.to_uppercase();
-    alias == "LOCAL"
-        || alias.starts_with(".LOCAL.")
-        || alias.starts_with(".VERSION.INI.")
-        || matches!(
-            alias.as_str(),
-            ".SYNC"
-                | ".LANSPREAD"
-                | ".LANSPREAD.JSON"
-                | ".LANSPREAD.JSON.TMP"
-                | ".LANSPREAD_OWNED"
-                | ".SOFTLAN_FIRST_START_DONE"
-                | ".SOFTLAN_GAME_INSTALLED"
-                | "INSTALL_INTENT.JSON"
-                | "INSTALL_INTENT.JSON.TMP"
-        )
-}
-
 fn validate_entry_shape(
     shapes: &mut BTreeMap<String, EntryShape>,
     alias_path: &str,