Register IPython's built-in magics lazily - #15340

Starting a shell imported all fifteen modules under IPython.core.magics
and instantiated every Magics class in them, even though a session
typically uses a handful of magics at most. Most of that cost is not the
class bodies but what they drag in: urllib and ast for %edit, cProfile,
pstats and timeit for %timeit, plus the eighteen @magic_arguments()
decorations that each build an argparse parser and format its help at
import time.

Only the magic *names* are now known up front, from hand-maintained
tables in IPython.core.magics._table, and the module implementing a magic
is imported the first time that magic is looked up. Registering a name
installs a LazyMagic placeholder: listing magics, completing them and
membership tests only read the table's keys and stay cheap, while
calling a magic -- or touching any attribute of one -- imports the
module, registers the real Magics instance in its place and delegates.
The same mechanism is available to third parties through
MagicsManager.register_lazy_class(), which, unlike the existing
lazy_magics config, does not require packaging the magics as an
extension.

This takes roughly 30 ms off `import IPython` and 20 ms off
import-plus-shell-startup.

Keeping the tables honest is tests/test_magic_table.py: it imports every
magics module, instantiates every Magics subclass, and fails -- printing
the corrected table -- if anything is missing, stale or attributed to
the wrong class. It also checks that starting a shell still imports no
magics module at all.

Places that need the whole picture rather than one magic keep working:
%config and its completions load everything first, so the list of
configurable classes is unchanged, and registry["ExecutionMagics"]-style
lookups load the class on the miss. init_magics no longer runs %colors
purely to trigger its observer, since that alone would import the basic
magics on every startup.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01BqGDoRyv11kHsBvvFQD4T4