fix(runtime): stop the cooperate checkpoint clobbering the last-error slot

[slepp]

Why

The actor scheduler's cooperate checkpoint used require_current_context(), whose null-context path has a side effect: it writes the EXECUTION_CONTEXT_NOT_INSTALLED sentinel into LAST_ERROR. That write clobbered genuine diagnostics — for example a real process launch failure — before stdlib callers could read them, so try_run_argv callers saw the context sentinel instead of the actual OS error text.

What

• hew-runtime (native + WASM): hew_actor_cooperate in scheduler.rs and scheduler_wasm.rs now uses the non-clobbering current_context() probe. The null check is semantically identical; only the spurious LAST_ERROR write is gone. Cooperate's contract is its return code — it is deliberately not a typed-error emitter, and the checkpoint never fabricates an error into the slot.
• Test contract fix: two tests had encoded the clobber as a contract. task_scope.rs's test is rewritten to assert the intended behaviour (a clear slot stays clear across a no-context cooperate, and a real diagnostic set before the checkpoint survives it); the execution-context catalogue case now documents cooperate as leaving the slot untouched.

Test

• New compiled regression test_try_run_argv_missing_command_preserves_real_error in tests/hew/process_test.hew: asserts the surviving error contains the real "failed to execute" text and not the sentinel. Verified red against the pre-fix runtime, green with the fix.
• cooperate_without_execution_context_preserves_last_error (task_scope.rs): covers both negative-space conditions — no fabricated error on a clear slot, and a pre-set diagnostic surviving the checkpoint.
• Full workspace nextest, the compiled-suite ratchet, and clippy/fmt are clean; the .hew regression was run three times to rule out order sensitivity.

Quality Checklist

• PR title, body, and commit messages avoid internal-only orchestration/model vocabulary
• No new .ok()? or unwrap_or_default() in codegen without // JUSTIFIED comment
• New allocations have cleanup paths for sync, async, and actor shutdown contexts
• Serialization changes include round-trip encode/decode tests
• New runtime features have WASM implementation or // WASM-TODO(#NNN): marker (with issue ref, e.g. #1451), and new hew_* exports are classified in scripts/jit-symbol-classification.toml
• No duplicated logic — checked for existing helpers before adding new ones

Out of scope

• The workspace-wide audit of other last-error readers (fs/stream/channel/net) — this fix is confined to the cooperate checkpoint.
• The Unix-only printf/kill fixture portability cleanup in the process tests.
• The 8 remaining require_current_context() diagnostic callers are correct as-is (their readers want the sentinel) and are unchanged.

Fixes #2506