feat: observer-based recording and stable animation for the canonicalizer

[GrovyleX]

Closes #199. An alternative implementation of the graph-transformation animation (see also #207).

Reintroduces the animation feature by wrapping the current Canonicalizer with an observer
(as suggested in the issue), so the recorder only watches and never changes the algorithm.

Demo

Smooth on a long leg (algebra a17, n=10). Positions and distances stay fixed as the graph
grows; a vertex moves only when the algorithm actually restructures its leg:

A-type 4*so(5) and B-type sp(4) worked examples:

What's included

• Recording layer (helpers/_recording.py): RecordGraph / FrameRecord / FrameGraph /
  recording_graph, capturing every event (lighting, contraction, attach, p/q, removals,
  replacements, dependents, leg manipulation).
• Observer (classifier/observer.py): typed CanonicalizerEvent enum + EventManager.
  The base Canonicalizer emits guarded notifications and is a no-op when nothing subscribes,
  so behaviour and all existing tests are unchanged.
• RecordingCanonicalizer (classifier/recording_canonicalizer.py): drop-in subclass,
  mirrors the existing TrackedCanonicalizer idiom; a recorded run classifies identically to
  a plain one.
• Renderer (helpers/drawing.py): per-role colours, an interactive to_jshtml player
  (play / pause / step / slider), and a stable incremental layout.
• Docs: restored A-type and B-type walkthroughs in classification.rst with embedded
  players and a swatch colour legend. Animations are generated at build time by conf.py, so
  no media is committed. docs/generate_animations.py regenerates them locally.

Notes on the review feedback raised so far

• Smooth, no-jump animation: the layout is persistent and incremental. A vertex keeps its
  position, spacing is constant, long legs fold into rows, and the viewport is frozen. On
  a17, n=10 (51 frames) only 8 frames have any motion, each a genuine restructure, ending in
  a clean star.
• Algebra in the terminal title: e.g. Canonical graph of type B3: su(1024).
• Distinguishable swatch legend: reuses the existing .color-legend / .swatch CSS.
• Interactive control bar: to_jshtml(default_mode="loop").
• Commented geometry: the new layout is documented block by block.
• Rebased on current main (post low-rank-isomorphism); classification.py is untouched.

Testing

• New tests/test_recording.py: frame invariants, layout stability (growth / relabel / fold),
  and recorded-vs-plain algebra equality across all G_LIE two-local algebras.
• Full suite: 966 passing. ruff, pylint (10/10), mypy src/paulie/application, and
  sphinx-build -W all green.