Fold hooks: JSON-RPC summaries for large added folds, concurrent streaming

[sidkmenon]

Summary

Adds a user-configurable fold hook that replaces fold placeholders with richer text, such as model-written pseudocode, before each streaming file event. Along the way, streaming now diffs files on a worker pool and emits them as they finish.

• Worker pool. --format ndjson diffs --jobs files at once (default 16) on a rayon pool and emits results in completion order. Clients lay out slots from the manifest already on start. 5x faster on a 1382-file debug-build diff.
• Fold hook. A [folds.hook] table names a trusted command. diffr starts it once per invocation as a JSON-RPC 2.0 server on a loopback port (DIFFR_HOOK_PORT), waits for it to listen, and calls summarize once per file with that file's large novel folds on the after side. The worker diffing that file blocks on its own reply; in-flight calls multiplex on a typed jsonrpsee client over a small tokio runtime. Failures keep the diff and add hook_error to the file event.
• Wire. Every fold gains a nullable summary beside its placeholder; the TUI schema accepts it. No rendering yet.
• Reference hook. examples/hooks/summarize.py: an aiohttp + jsonrpcserver server that asks Gemini 3.8 Flash, thinking off, for Python-style pseudocode, concurrently on one asyncio loop with a shared httpx client.
• Config-relative paths. Hooks run in the directory containing the config file, so one config works from any checkout. DIFFR_WORKSPACE carries the repo root.

Design notes and the full protocol are in docs/streaming.md.

Test plan

• cargo test --bin diffr (173 passed), including hook unit tests against a stdlib JSON-RPC server: id routing under 8 concurrent callers, timeout, startup exit, error objects, invalid responses
• tests/streaming/check.py: hook success, error object, startup exit, unstartable command, config outside the repo with a relative script path, --jobs 1 ordering
• bun test in tui/ (42 passed)
• End to end against Gemini: 13 folds across 4 files in about 5s wall, no hook errors
• cargo fmt, cargo clippy, ruff format and ruff check clean on touched files

AI disclosure

Implemented with Claude Code per AI_POLICY.md; every change was read and verified by a human-directed session, and the design was iterated in review.

https://claude.ai/code/session_015ZLcD4fU7YTe4CTcB1Q7oN