fix(engine/protocol): pool stability fixes for morph-kgc and postprocessors

[rpanfili]

Fixes three bugs introduced in v8.0.0 that could cause silent failures or resource starvation under concurrent load, plus a small observability improvement.

Changes

• Engine — BrokenProcessPool recovery: when the morph-kgc subprocess pool dies (e.g. OOM-killed worker), the engine now recreates it and retries instead of propagating the exception.
• Protocol — wrong pool size variable: mapping_pool_size was shadowed by the wrong local variable when constructing the ThreadPoolExecutor, causing it to be sized incorrectly.
• Protocol — postprocessor pool cap: postprocessor pool is now capped at 75% of pool_size (min 2) so it can't consume all workers and starve the mapping stage.
• Logging: url-fetch and postprocessor concurrency levels are logged at startup for easier tuning.