fix: cookbook cache invalidation — use max(recipe, source) timestamps

Bug: src/cook/cook_build.rs:268 only bumps source_modified when
recipe.toml is STRICTLY newer than source. In normal workflow:
  1. Recipe edited at T1 (adds patch to patches array)
  2. Source re-fetched at T2 > T1 (during make r.<recipe>)
  3. Build caches stage at T3
  4. Next build: recipe(T1) > source(T2) = FALSE → edit ignored

Fix: source_modified = source_modified.max(recipe_modified);
always considers recipe timestamp regardless of relative ordering.

Root cause of kernel rebrand not taking effect was ALSO a missing
.git/HEAD in the source tree (cookbook skips patches for release
archives). Re-fetch with 'repo --allow-protected fetch kernel'
restored the git repo and enabled patch application.

Verified: 'RedBear OS starting...' appears in QEMU boot log.

src/cook/cook_build.rs

@@ -265,9 +265,7 @@ pub fn build(
 
     let mut source_modified = modified_dir_ignore_git(source_dir).unwrap_or(SystemTime::UNIX_EPOCH);
     if let Ok(recipe_modified) = modified(&recipe_dir.join("recipe.toml")) {
-        if recipe_modified > source_modified {
-            source_modified = recipe_modified
-        }
+        source_modified = source_modified.max(recipe_modified);
     }
 
     let deps_modified = modified_all_btree(