Rename game from Whack Hare! to Nyuller

All references updated:
- src/build.sh: PRG=nyuller.prg, D64=nyuller.d64, disk name 'ny',
  all grep/pgrep patterns updated, all comments updated
- All 10 header files: include guards WHACK_HARE_* → NYULLER_*
- C source comments: 'Whack Hare!' → 'Nyuller'
- README.md: updated build command output path
- GAME.md: title updated
- tasks.md: all whack_hare.prg → nyuller.prg
- src/AGENT_CONTEXT.md: project name updated
- tools/convert_screens.py: project name updated

Build artifacts renamed: whack_hare.* → nyuller.*, whack.d64 → nyuller.d64

Note: title screen image (source_images/screen_title.png) still shows
'WHACKED' logo — that's a visual asset, not a code reference.

tasks.md

@@ -1,4 +1,4 @@
-# Tasks — Whack Hare! implementation plan
+# Tasks — Nyuller implementation plan
 
 A phased, testable plan. Each phase ends with a *verify* step that
 proves the phase works. Each phase is one or more commits. Stop at any
@@ -171,7 +171,7 @@ state.
 - `cd src && ./build.sh -e` displays the title screen in the
   oscar64 built-in emulator for 5 seconds (or until fire is pressed)
   then exits.
-- (Optional, interactive) `x64 build/whack_hare.prg` in a real
+- (Optional, interactive) `x64 build/nyuller.prg` in a real
   terminal session displays the title screen on real timings.
   Compare visually to `source_images/screen_title.png`.
 - The .map file shows our code is in $0900-$1100-ish, well within
@@ -304,7 +304,7 @@ random source.
 **Verify:**
 - The game still works in the oscar64 built-in emulator
   (`build.sh -e`).
-- (Optional, interactive) Launch `x64sc build/whack_hare.prg`
+- (Optional, interactive) Launch `x64sc build/nyuller.prg`
   in a real terminal to confirm the game runs at cycle-exact PAL
   timing (50.125 Hz). The state transitions happen on the right
   raster lines. The 50 Hz tick is rock-solid.
@@ -363,7 +363,7 @@ becomes audible.
   - GAMEOVER: longer fanfare.
 - The cues are recognizable and feel right (timing, volume).
 - (Optional, interactive) Run the audio cues under `x64sc
-  build/whack_hare.prg` in a real terminal to confirm the
+  build/nyuller.prg` in a real terminal to confirm the
   SID envelope generator timings are right; the oscar64 emulator
   is fast but not always bit-accurate on SID timing, and the
   `x64sc` cycle-exact path is the one that matches real hardware.