fix(indexing): keep gateway responsive during blocking indexing phases - #1763

Indexing currently blocks gateway health/status requests during discovery, grammar loading, import pre-scans, resolution and synchronous graph writes. Move the Tree-sitter phases to workers while keeping writes sequential. Run the blocking SCIP pipeline, including its CLI and thread-local parser augmentation, in one worker; cancellation checkpoints stop subsequent phases after the current blocking call returns.

Before loading an uncached language, use language-pack prefetch. In the pinned v1.20.0 binding, prefetch releases the GIL while get_language does not, so merely moving get_language to a thread is insufficient. Keep alias mapping and language caching; surface prefetch failures through #1761's initialization errors. Document offline cache preparation and the limitation that in-flight native calls/writes cannot be interrupted or rolled back (#829 remains open).

Validation: 14 new regressions exercise the real /health route while Tree-sitter discovery/pre-scan/resolution/writes and SCIP CLI/supplement/writes are blocked, cold-cache failure without per-file retries, cancellation before subsequent writes, and alias/cache behavior. The parser/utils/API/indexing regression run passes 520 tests. Graph-write/resilience/convergence coverage passes 64 tests with 1 skip. After adding the failed-write cancellation guard, the final focused liveness/SCIP run passes 22 tests. Ruff and git diff --check pass. The published Git tree matches the tested local tree.

Stacked on #1761, targeting its branch to keep the review diff focused. Retarget to main after #1761 merges and require the main-target CI workflows before merge. Current repository rules still require an independent approving review.

Refs #1745. This PR does not resolve the Ladybug CONTAINS defect in #1730.

Upstream binding reference: https://github.com/xberg-io/tree-sitter-language-pack/blob/v1.20.0/crates/ts-pack-core-py/src/lib.rs .