feat: expose canonical independent-spawn substrate

[zackees]

Implements #1202 and supplies the producer half of kernal-api#189.

• preserves the existing independent scheduler backends and exposes the canonical public option/result API
• restores retained process compatibility operations under producer-owned types
• keeps kernel-substrate minimal; independent spawning is an explicit separate feature
• prepares 4.10.13 for the downstream registry migration

Validation:

• cargo +1.95 test -p running-process --lib (590 passed)
• python -m ci.lint source-policy and substrate-contract stages passed; the full lint command exceeded the local runner time budget while running fixture consumer checks
• soldr cargo check --workspace --quiet
• uv run --no-project --module ci.version_check