[codex] Revive doeff-vm test layer

[proboscis]

Summary

• Replaced the stale packages/doeff-vm/tests/test_pyvm.py legacy API layer with current bridge contract tests for PyVM, low-level doeff_vm.WithHandler, doeff.run(...), handler installers, scheduled(...), and vm_live_counts().
• Updated package-local export, memory/liveness, and Rust source-guard tests to match the current public API.
• Wired packages/doeff-vm/tests into the standard pytest path via pyproject.toml testpaths.
• Made packages/doeff-vm/src/pyvm.rs VMError/Value matches explicit so the refreshed source guard does not rely on catch-all arms.

Deleted stale tests grouped by removed feature

• Removed module-level doeff_vm.run / doeff_vm.async_run, RunResult, run_with_result, raw_store, and env/store result-shape tests.
• Removed doeff_vm.state, reader, writer, scheduler, put_env, state_items, env_items, logs, and PyStore sentinel/store API tests.
• Removed old async step-driver tests for start_program, step_once, feed_async_result, and feed_async_error.
• Removed legacy scheduler effect pyclass tests (SpawnEffect, GatherEffect, CreatePromiseEffect, old race/promise imports) and rewrote scheduler coverage through scheduled(Spawn/Gather).
• Removed legacy tag constants, DoeffGeneratorFn, FlatMap, Discontinue, no-arg Delegate, and ResumeContinuation assertions that no longer describe the public API.

Rewritten coverage retained

• PyVM().run(...) for current @do programs and low-level DoExpr nodes.
• Python handler semantics for Resume, Transfer, ResumeThrow, Pass, nested WithHandler, and GetHandlers(k).
• Current state(), reader(), and writer() Program -> Program installers.
• Current scheduler path via doeff.run(scheduled(...)).
• Current package exports and liveness diagnostics through vm_live_counts() / arena_stats().

Verification

• uv sync --reinstall --group dev passed.
• PATH="$HOME/.local/bin:$PATH" make sync passed and rebuilt packages/doeff-vm with maturin develop --release.
• uv run python -m pytest packages/doeff-vm/tests -q -> 28 passed.
• uv run python -m pytest tests/core/test_vm_legacy_tag_parser_fail_loud.py -q -> 6 passed.
• uv run python -m pytest tests -q -> 880 passed, 90 skipped.
• uv run ruff check packages/doeff-vm/tests packages/doeff-vm/doeff_vm -> passed.
• cargo fmt --manifest-path packages/doeff-vm/Cargo.toml --check -> passed.
• PATH="$HOME/.local/bin:$PATH" semgrep --config .semgrep.yaml --baseline-commit origin/main doeff/ packages/ --error -> 0 findings introduced by this commit.

Note: full PATH="$HOME/.local/bin:$PATH" make lint-semgrep still fails on 77 pre-existing repository findings; the baseline semgrep check above reports no new findings from this PR.

Refs: vm-pyvm-stale-test-layer