Cap concurrent in-flight LLM calls during indexing via a shared
semaphore (bounded_gather), so a many-node document no longer schedules
one socket per node and exhausts the process fd limit (Errno 24).
Make the per-index max_concurrency override correct under concurrency:
- Scope IndexConfig(max_concurrency=...) to the build_index call via a
ContextVar (max_concurrency_scope) instead of mutating a process
global. A one-off value no longer sticks as the new default, and
concurrent indexing of other documents isn't affected.
- Propagate the context through _run_async's worker-thread fallback so
the override survives the sync-over-async thread hop.
- set_max_concurrency() stays as the explicit process-wide setter.
Also stop `from .utils import *` leaking a `config` name (SimpleNamespace
alias) that shadowed the real pageindex.config submodule for the
page_index modules; the alias is now `_config`.
Adds regression tests for cap enforcement, scope stickiness/isolation,
worker-thread propagation, and the config-namespace fix.
Claude-Session: https://claude.ai/code/session_01Kx5DgKbhK1N8autqXH8SmS
