build: add run_magic*.sh launchers to run magic from the build tree

Four configure-generated launch scripts let you run magic straight from the
build directory without `make install`:

  * run_magic.sh          -- GUI magic (Cairo/X11)
  * run_magicnull.sh      -- batch / no-GUI (-dnull)
  * run_magic_gdb.sh      -- magic under gdb
  * run_magic_valgrind.sh -- magic under valgrind

magic resolves its runtime files through a single $CAD_ROOT root
($CAD_ROOT/magic/{sys,tcl}), but in the build tree they are scattered across
build/ (generated tech, .magicrc, magic.tcl, tclmagic, execs) and source/
(colormaps, dstyles, glyphs, fonts, tcl scripts).  On first run each launcher
self-stages ${builddir}/CAD_DIR as a symlink tree in that layout (no copies,
always current) via the shared scripts/magic_run_common.sh, exports
CAD_ROOT=${builddir}/CAD_DIR, and execs the right binary.  This pairs with the
preceding commit that made the C launchers honor CAD_ROOT.

magic_stage() is idempotent and re-runs on every launch.  Most staged entries
are files, but the bitmaps entry links a *directory*, so it uses `ln -sfn`
(--no-dereference; also accepted by BSD ln): a plain `ln -sf` would, on a second
run, dereference the existing symlink-to-directory and drop the new link *inside*
the target -- a magic/bitmaps/bitmaps self-loop in the source tree that trips up
find/tar/cp -r.

Verified: ./run_magicnull.sh self-stages (30 sys + 24 tcl symlinks) and runs --
`tech load scmos` reports 13 planes -- straight from the build dir.  gitignore
covers the generated scripts and the CAD_DIR tree.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

scripts/run_magicnull.sh.in

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+#
+# Launch magic in batch / no-GUI mode (-dnull) straight from the build tree.
+# Generated by configure from scripts/run_magicnull.sh.in.
+#
+# Interactive use sources the Tcl startup (magic.tcl) automatically from CAD_ROOT.
+# For a non-interactive script piped on stdin, source magic.tcl yourself first:
+#   printf 'source %s/magic.tcl\n<commands>\n' "$CAD_ROOT/magic/tcl" | ./run_magicnull.sh
+set -eu
+here="$(cd "$(dirname "$0")" && pwd)"
+. "$here/magic_run_common.sh"
+
+magic_require_built
+magic_stage
+
+exec "$MAGIC_TCL/magicdnull" -dnull -nowrapper "$@"