Tidy up for 0.9.3: Shagu overlap warning, readme rewrite

- Warn once in chat when ShaguTweaks Debuff Timer draws on target debuffs.
- Use STANDARD_TEXT_FONT for timers and stack counts.
- Clean up comments and dead code; add .gitattributes and .gitignore.
- Rewrite README (ClassicAPI version table, ShaguTweaks compatibility);
  move history to CHANGELOG.md.
- Tests: 19 scenarios, no machine-specific paths.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019W3rwutyoLNHxXPycYWSMc

VALIDATION.md

@@ -1,8 +1,8 @@
-# TargetFrameBuff v0.9.2 validation
+# TargetFrameBuff v0.9.3 validation
 
-17 simulated Lua 5.1 regression scenarios passed, plus XML parsing. These cover
+19 simulated Lua 5.1 regression scenarios passed, plus XML parsing. These cover
 old/native clients, aura reordering, overflow, settings, sweep lifecycle, delta
-refreshes and full updates, target/pet/spellbook changes, reused delta payloads,
+refreshes and full updates, the Debuff Timer overlap warning, target/pet/spellbook changes, reused delta payloads,
 no polling in delta mode, and native-number/flash suppression.
 
 ClassicAPI 1.17.10+ is detected through GetAuraDataByAuraInstanceID, introduced with
@@ -16,4 +16,5 @@ New Clear and SetDrawBling methods are optional, preserving older versions.
 
 Not tested inside a running WoW client. Verify recasts, target switches, auras
 expiring, and sweeps with countdownForCooldowns enabled. Keep ShaguTweaks Debuff
-Timer disabled to prevent overlapping addon countdowns. No FPS gain is claimed.
+Timer disabled to prevent overlapping addon countdowns (a chat warning appears
+if it is detected). No FPS gain is claimed.