perf(skins): skip redundant skin parsing, base reloads and theme writes

Skin handling did more work than it needed:
- the bundled-skin scan fully parsed each of the 9 bundled skin files to
  read name, author, dark and images from their first 327-345 bytes. The
  build copies the rest of ui.toml into every skin, so each file is
  51-54 KB;
- setActiveSkin() reloaded ui.toml right after main() had loaded it, a
  second full parse with nothing merged in between;
- in the Windows single-file exe, every getBundledSkinsDirectory() call
  (skin refresh, the dragonx entry, each image-dir lookup) re-extracted
  all embedded themes with a blind overwrite: more than once per launch,
  and again on skin changes.

Now:
- bundled skins are parsed only up to the first table header after
  [theme] other than [theme.images]. Anything unusual (no [theme],
  images defined later, a parse error in the slice) falls back to the
  full parse, and user skins are still parsed in full;
- UISchema tracks whether its state is exactly the base (the last change
  was a base load and no hot-reload is pending), and setActiveSkin()
  skips reloadBase() in that case;
- the bundled-skins directory is resolved once per process (re-checked
  with exists()), and theme extraction rewrites a file only when its
  bytes differ, through the new resources::writeFileIfDifferent(). It
  compares bytes, not sizes, so a same-length palette edit still
  updates.

Measured on the Lite build (median of 5 runs):
- bundled-skin scan: 15 -> 1 ms;
- from "Initializing ObsidianDragon" to the activated skin: 29 -> 12 ms;
- ui.toml is loaded once per launch instead of twice, and the light skin
  merges its overlay exactly once.

A Python mirror of the header-slice parse agrees with a full parse on
name, author, dark and images for all 18 source and build-expanded skin
files, and full screenshot sweeps over all 10 skins match the previous
build within noise. New unit tests cover writeFileIfDifferent(): missing
file, identical bytes (mtime untouched), same-length change, and a
longer file truncated.

Behaviour changes:
- a bundled skin with a TOML error after its header is now listed
  instead of skipped; the error surfaces only when it is selected;
- deleting a single file inside the extracted bundled-themes dir
  mid-session is repaired only at the next launch;
- the "Extracted N embedded bundled themes" count includes files that
  were already current.

Not verifiable here: Linux has no embedded themes, so the new
writeFileIfDifferent() ran only in the unit tests and the extraction
path was only compiled, in the Windows cross-build. That a second
Windows launch leaves the theme files' mtimes untouched is still to be
checked on Windows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui/schema/ui_schema.h

@@ -108,6 +108,10 @@ public:
      */
     bool reloadBase();
 
+    /// True while the loaded state is exactly the base file/string (no overlay merged since the last base load,
+    /// no pending hot-reload).
+    bool isPristineBase() const { return pristineBase_ && !dirty_; }
+
     /**
      * @brief Get the active overlay file path (empty if no overlay)
      */
@@ -337,6 +341,7 @@ private:
     std::filesystem::file_time_type lastModTime_;
     std::filesystem::file_time_type baseModTime_;   ///< Track base file changes when overlay active
     bool dirty_ = false;
+    bool pristineBase_ = false;   ///< last mutation was a base load (loadFromFile(basePath_) / loadFromString)
     bool fonts_changed_ = false;
 
     // Theme