Harden and shard test infrastructure

[yyjeqhc]

Summary

• inventory Git-tracked Rust tests across the full workspace with a self-tested, sanitized area/risk report
• harden Runner test synchronization, process lifecycle fixtures, polling-thread cleanup, and shared environment isolation
• deduplicate ordinary CI gates and shard Linux Rust tests by complete Cargo package groups while preserving the historical aggregate test status and release-readiness boundary
• clarify the model-facing git_diff_hunks continuation contract: continuation tokens are scope-bound, so callers must replay the original diff scope and paging inputs unchanged

Review notes

The test cleanup is intentionally test/CI/tooling focused; no production Runner behavior was changed by the cleanup commits. The final git_diff_hunks change is schema/description-only and does not alter continuation encoding, fencing, or runtime behavior.

Validation

• bash scripts/test_inventory.sh --self-test — 2 passed
• bash scripts/test_inventory.sh — passed on the repository inventory
• cargo test -p webcodex-runner — 744 passed, 0 failed, 3 ignored
• cargo test -p webcodex tool_specs_describe_default_coding_loop_preferences — 1 passed, 0 failed
• cargo fmt -- --check — passed
• git diff --check / staged diff check — passed

Base reviewed against 37f23e67ebfb03f18ccefd613b95d231e74f1438 (Add Workflow Session message observation delta (#126)).