Captured compiler diagnostics

std/diagnostics.diagnostics_decode(tools, request) decodes captured output. It starts no process and reads no file. Hosts keep command execution, permissions, capture limits, and presentation in their existing adapters.

The request supplies format, stdout, stderr, nullable exit_code, completed, and capture_complete. Optional source_text maps Harn byte spans to source lines. Formats are cargo-json, go, generic, positional, ruff, shellcheck, scala3, php-lint, zsh, and harn-json.

The result includes located diagnostics, measured, reason, and nullable error_count. Only a completed process with an exit status and complete capture can measure errors. Unknown formats, malformed JSON, and failed commands without recognized errors remain unmeasured. Partial findings remain visible, with a null error count. A documented empty JSON array from a completed zero exit measures zero errors. Text formats use the compiler exit status to establish a clean check.

tools.run_build_command uses this same decoder and reports whether its diagnostics were measured in diagnostics_complete.