[chagui]

Adds opt-in consumption of Bazel Progress BEP events as OTel log records. Each non-empty stream produces one record at INFO severity, stamped with bazel.progress.{stream,bytes,truncated,opaque_count}. When the body exceeds progress.max_chunk_size the receiver clips at a UTF-8 rune boundary (never producing invalid encoding) and bazel.progress.truncated=true. max_chunk_size: 0 disables the cap; config validation rejects negative values.

Default is progress.enabled: false to avoid surprise log volume — Bazel builds with --build_event_publish_all_actions produce a lot of Progress. Operators opt in explicitly, and the bundled 64 KiB chunk size keeps the worst-case per-record payload bounded once they do.

Review-driven design choices

• Severity is INFO on both streams. Bazel routes normal-build chatter (Loading:, Analyzing:, Build completed successfully) through stderr — mapping stderr to ERROR would page on every green build. Operators who need elevation can stack a transformprocessor downstream.
• Emission order is stderr-first, matching the BEP spec (build_event_stream.proto:303-304: "stderr has been emitted before stdout if both are present"). Both records on a single Progress event share the same opaque_count so consumers can pair them and reconstruct chunk order.
• Rune-safe truncation. The body clip backs off to a UTF-8 rune boundary using a new shared truncateUTF8 helper (also adopted by reconstructCommandLine to dedup the existing manual loop).
• Always-on bytes and truncated attributes. Both are stamped on every record so downstream queries can rely on attribute presence rather than guarding for absent keys. The previous original_bytes-only-on-truncation contract is gone — replaced by bytes (always = original byte length).
• Post-flush gate. handleProgress checks state.flushed, matching the gate every other event handler uses; late Progress arriving after BuildFinished is dropped silently rather than appearing in a separate consumer batch.
• emitLog / emitProgressLog deduplication. Both now share a newLogPayload helper for the resource/scope boilerplate, eliminating the duplicated service.name="bazel" literal and the duplicated record-construction logic.

Memory bounding — explicit non-goal

This PR truncates the log record body, not the inbound gRPC payload. The full Progress message is still allocated by proto.Unmarshal before the handler runs. See docs/adr/0002-large-payload-handling.md and the concrete follow-ups #64/#65/#66 for the heap-bounding story.

ACK back-pressure — known trade-off, follow-up filed

Routing Progress through the existing synchronous ConsumeLogs path widens an existing back-pressure surface: a stalled logs pipeline now holds BES ACKs on the highest-volume BEP event class. The pattern pre-exists for every other handler, so fixing it correctly is project-wide. Filed as #72 (async/bounded logs path).

Deferred to follow-ups

• Async/bounded logs path to avoid BES ACK back-pressure #72 — async logs path so logs-pipeline back-pressure doesn't gate BES ACKs
• Per-stream Stdout/Stderr toggles for Progress logs #73 — per-stream progress.stdout / progress.stderr toggles

Closes #60.