fix(colgrep): responsive Ctrl+C + non-resetting build progress while indexing

[raphaelsty]

Fixes two indexing UX issues reported on a large fresh build. Neither corrupted the index — both are about progress display and interrupt responsiveness — and crash-safety/resumability is unchanged and verified.

1. Progress bar restarted at 0 every ~4096 units

The resumable build encodes in BUILD_CHECKPOINT_UNITS (4096) batches, each running a fresh pipeline. The metadata stage set the bar via set_position(completed_units) from a per-batch counter starting at 0, so the bar climbed to ~4096 then jumped back. Switched to pb.inc(delta) so the shared bar accumulates across batches to the true total.

2. Ctrl+C appeared to do nothing while indexing

• No feedback: the acknowledgement only printed inside a critical section, so a Ctrl+C during the long encoding phase was silent. Now the first interrupt is always acknowledged.
• Slow to act: interruption was only polled at chunk/batch boundaries, so the whole in-flight pipeline drained before stopping (many seconds). The encode stage now bails between chunks, and a new encode_prepared_document_batches_cancellable checks the flag between batches, so a Ctrl+C lands within ~one model forward pass. Measured stop latency: full-queue drain → ~1 s.

The hard floor is one ONNX session.run() (an uninterruptible FFI call); the residual ~1 s is the clean shutdown finalizing in-flight work (k-means seed, the accumulated index write, queued metadata inserts) so the index stays consistent.

Safety / resumability (verified)

Index writes stay inside a CriticalSectionGuard, state.json is checkpointed per batch, and build_resumable trims any partial mid-batch write on resume. Interrupt+resume tested on a 30,000-unit build, interrupted both before and after the first checkpoint:

• stops in ~0.7–1.5 s with the feedback message,
• resume completes,
• final index = exactly 30000 docs, 30000 distinct _subset_ (no duplicates, no loss), and unique-token searches resolve to the correct file (4/4).

make ci-quick (fmt + clippy + 562 colgrep tests) passes.