feat(compute): Morph microVM backend + pre-warm pool for the computer plane

[mercurialsolo]

What

Adds Morph (morph.so) as a 5th compute-plane backend (local/modal/e2b/
daytona/morph), plus a pre-warm pool that eliminates cold-start
latency. This is the legitimate latency/reliability use of Morph's instant-
snapshot microVMs — explicitly not anything to do with bot-detection.

Pieces

• gym/morph_impl.MorphComputerImpl (extends RemoteComputerImpl, mirrors
  E2BComputerImpl): start an instance from a snapshot → expose HTTP service →
  await /health → delegate the screenshot+xdotool wire. Two modes:
  • provision — fresh instance, torn down on close()
  • claim — a pre-booted instance+base_url from the pool (pool owns its
    lifecycle, not torn down by the run)
    All morphcloud-SDK calls are isolated in injectable, getattr-guarded helpers
    → tests use a fake SDK, zero real API calls.
• gym/morph_prewarm_pool.MorphPrewarmPool: keeps size booted +
  health-checked instances ready; acquire() pops one instantly + refills in
  the background; release()/drain() stop instances (single-use — a run
  mutates VM state). refill_async=False for deterministic tests.
• computer_client: backend="morph" dispatch (claim from morph_pool
  when set, else provision from morph_snapshot_id); config fields threaded
  through resolve_for_executor.
• pyproject: optional morph extra.

Tests

test_morph_impl.py (5) + test_morph_prewarm_pool.py (5): provision / claim /
health-timeout-teardown / missing-creds; pool top_up / acquire-hot /
cold-miss / release / drain. 54 compute-plane tests green; ruff clean.

Activation (operational follow-up — not in this PR)

This backend is inert until selected (default stays local/modal). To turn
it on: build a Morph snapshot of the computer-plane image (Xvfb + Chrome +
screenshot/xdotool service on :8000 with /health), set
MANTIS_MORPH_SNAPSHOT + MORPH_API_KEY, construct a MorphPrewarmPool at
server startup, and set backend="morph". Verify the exact morphcloud SDK
method names against the installed version first — they're isolated in the
helpers, so any drift is a one-line fix.

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