perf(task): lazy task registry backed by a static AST index

[geng-haoran]

list_tasks() / get_task_class() imported every module under every task package
(pkgutil.walk_packages + import_module): with RoboVerse that is ~730 modules, two 20k-line
generated files, and every import-time side effect (asset downloads, optional-dependency warnings)
before the first lookup — 7.3 s, and a failure blast radius that grows with each task family.

• metasim/task/_static_index.py: walks the package directories (no imports — pkgutil.walk_packages
  would import each sub-package __init__, some of which eagerly import their whole family), parses
  each file with ast, and records every register_task(...) call with string-literal arguments as
  name -> module. Calls with non-literal arguments (loops) flag the module as dynamic. Results are
  cached per file (mtime + size) in $METASIM_CACHE_DIR/task_index.json.
• registry.get_task_class imports only the registering module; unknown names fall back to the
  old import-everything discovery so the KeyError still lists import failures.
  registry.list_tasks returns the index plus dynamic modules' names. The cached index is keyed by
  the package configuration and rebuilt when it changes (env vars, cwd); an emptied registry re-runs
  discovery. METASIM_TASK_DISCOVERY=eager restores the previous behaviour.

Measured on RoboVerse main (6051 tasks): index cold 1.08 s / warm 0.04 s; get_task_class 0.01-0.1 s
(mjlab 0.9 s = its own imports); list_tasks 4.9 s (3.5 s of it is importing
maniskill.native_tasks, which registers 5360 names in a loop) versus 7.3 s before. All 2898
decorators in RoboVerse are string literals.

Tests: metasim/test/test_task_registry_lazy_general.py (6 tests on a synthetic package: single
module imported per lookup, list without imports, eager fallback + failure reporting, env var,
cache reuse/invalidation). pytest -k general -> 492 passed; -k mujoco unchanged.

Co-Authored-By: Claude Fable 5.1 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_017i6VtKoovBNed815mWFqxw

🤖 Generated with Claude Code

https://claude.ai/code/session_017i6VtKoovBNed815mWFqxw