[vishr]

What changed

Adds FanQL, a compact text form of Fanout's structured query, so people and
agents can query telemetry without an LLM key or raw SQL:

logs where service:api severity>=error | count() by service every 1m

• Hand-written lexer and recovering parser, resolver against the telemetry
  catalog, and lowering to the existing structured query. A canonical
  printer lifts stored queries back to text through the engine parser.
• Conditional aggregates (count_if, …) and measure arithmetic for spans
  and logs, using one shared measure grammar with query budgets and a
  disclosed 20-sample ranking floor.
• Diagnostics report every independent problem in source order, with field-
  specific repairs and one-click fixes (span, fix_its).
• Completion is position-aware, with bounded, coalesced schema fetches and
  absolute or relative windows.
• HTTP: POST /api/fanql/parse, POST /api/fanql/format,
  POST /api/fanql/complete, GET /api/fanql/reference.
• MCP: query_telemetry accepts optional FanQL text (syntax and examples
  inline). The reference is published as fanout://fanql/reference and in
  the site docs, generated from the same registries the engine uses.
• UI: AI controls are hidden when chat is disabled (supersedes feat(ui): hide AI controls when chat is disabled #395).
• Design: docs/specs/2026-10-29-no-llm-ui-design.md (section 0).

FanQL lowers to the existing validated query, so the arbitrary SQL rules and
telemetry relations are unchanged. There is no language version field.

Verification

• just check (pre-push and CI)
• just test-race
• Corpus, round-trip, truth-table, fuzz and diagnostic-copy tests;
  latency budgets as opt-in benchmarks (just fanql-perf)
• Two-axis code reviews (standards and spec); all findings fixed with
  regression tests
• Generated HTTP, MCP and site references updated
• No credentials, private telemetry, host details, or enterprise-only source
• API and MCP contract additions are called out above; no migrations,
  ingest or release-contract changes