Phase 4: import hook / proxy for native-only packages

[zacharywhitley]

Background

Design: docs/native-execution-and-parallelism.md §4.1.
Depends on #1 and #2 (needs a working Tier 1 backend).

Make Tier 1 ergonomic: configured "native-only" packages import as proxy modules
whose calls route over offload.run.

Tasks

• meta_path finder in the WASM CPython that intercepts configured
  native-only package imports and returns a proxy module.
• Proxy module: attribute calls serialize (entry, args), invoke
  offload.run, deserialize the result.
• Configuration of the native-only package set (ties to env-id).
• numpy as the proof case.

Acceptance

import numpy as np; np.<call>(...) in the WASM interpreter transparently
executes in the Tier 1 backend and returns a correct result, for
call-with-serializable-args.

Out of scope

Transparent live-object proxying (ndarray views, callbacks, __getattr__
chains) — that is Phase 5.

[zacharywhitley]

Progress: import-hook mechanism landed (3469f8c)

The host-side core is in place and green in CI (reference-worker, 23 tests):

• importhook.py — install(packages, client) registers a meta_path finder so
  import <pkg> returns a proxy module; attribute calls offload
  <pkg>:<dotted.attr>(args, kwargs) through any offload client
  (SubprocessClient/MailboxClient/...) and return the decoded result. A remote
  exception is re-raised as the matching builtin type when one exists, else
  OffloadError. Hook.uninstall() restores sys.modules.
• test_importhook.py — proxied call, kwargs, reconstructed ValueError, and
  real-module restoration. math stands in for a native-only package, executed in
  a backend worker process (proving the import → proxy → offload → backend path
  end to end).

Remaining

• The numpy proof case — numpy installed in a backend environment (ties to
  a working Tier-1/Tier-P backend and env-id).
• Richer attribute handling (module-vs-attribute boundary, constants/values vs
  callables). Note: transparent live-object proxying is Phase 5: evaluate transparent object proxying (stretch) #5, out of scope here.