Phase 2: display the title screen (mmap, vic, screen swap)

Replace helloworld.c with main.c. Add memmap_setup/restore (banks out
KERNAL+BASIC+CHAR ROMs so $E000-$FFFF is free for the 8 KB bitmap and
$D000-$DFFF is I/O), show_screen(n) which copies the title .bin to
$E000 and the .attr to $D000 (screen memory in multicolor bitmap mode)
then flips the VIC into BMM=1 MCM=1 CSEL=1 RSEL=1 DEN=1 with bitmap
base CB13=1 and screen base VM13-VM10=4 ($D018 = 0x48), and
input_fire(port) which reads bit 4 of CIA1 $DC00/$DC01.

build.sh default target is now main.c -> whack_hare.prg (still
builds helloworld explicitly via the existing -c flag if needed).

Verified: ./build.sh -e runs the title screen in oscar64's built-in
emulator and exits cleanly. .map shows code at $0880-$09A6 and
embedded title data at $09A7-$2CCF, well within the 38 KB main
region. .prg is 9428 bytes, well under the 202-block LOAD"*",8,1
limit.

Concerns for Phase 3 (score bar): the score bar will overlay the top
8 pixel rows of the bitmap ($E000-$E13F, 320 bytes). show_screen()
will continue to copy the full .bin to $E000; score_render() will be
called immediately after and overwrite the top 320 bytes. This keeps
show_screen() dumb and lets the score bar be re-rendered on state
change without re-copying the whole 8 KB bitmap.

src/memmap.c

@@ -0,0 +1,13 @@
+#include "memmap.h"
+
+void memmap_setup(void)
+{
+	mmap_trampoline();
+	mmap_set(MMAP_RAM);
+	mmap_set(MMAP_NO_ROM);
+}
+
+void memmap_restore(void)
+{
+	mmap_set(MMAP_ROM);
+}