Prototype live compact execution log streaming

[OLI-OAI]

Summary

Prototype a dedicated live execution-log gRPC stream instead of routing logs through CAS ByteStream uploads.

This is stacked on the three commits in dzbarsky#35. The new protocol can begin before Bazel knows the final digest:

• add build.bazel.executionlog.v1.ExecutionLogService.Stream
• stream zstd-compressed compact execution-log bytes as records are emitted
• finish with SHA256 and byte count, validated by the receiver
• add --experimental_execution_log_stream_endpoint
• flush compact records through a 64 KiB / 100 ms batching layer
• keep the existing BEP artifact-upload path intact

Validation

• bazel build //src/main/java/com/google/devtools/build/lib/bazel:spawn_log_module
• bazel test //src/test/java/com/google/devtools/build/lib/util/io:io_tests --test_output=errors
• release-shaped build: bazel --batch --nohome_rc --nosystem_rc build --verbose_failures --compilation_mode=opt --jobs=2 --stamp --embed_label=9.3.0-live-prototype //src:bazel
• E2E with OpenAI receiver:
  • //api/botmail:botmail_cloudflare_email: 8 streamed actions/lane, 8 deterministic
  • //api/api_healthcheck:api_healthcheck_bin: 15,462 Bazel actions/lane, 10,266 streamed actions/lane
  • after batching: 246 gRPC data messages for ~9 MB compressed log

Related: dzbarsky#35