Spectre sink: Logging & output architecture — Serilog event core + Spectre.Console frame + NDJSON machine path #391

Summary

Replace the inherited NUKE console output with a two-layer model:

• Serilog owns the structured event stream (one set of log calls, fanned
  out to a human sink and a machine sink).
• Spectre.Console owns the rich frame (run header, end-of-run summary
  table), driven directly by the build executor — NOT routed through a sink.

Adds first-class NDJSON output so agents/tools can shell out to Fallout and
parse a clean machine stream.

Motivation

Supports the AI-native CLI goals: NDJSON streaming, describe --json,
structured errors, semantic exit codes. The structured-event model maps a
single set of log calls onto both a pretty console and a machine stream, so
human and agent consumers stay in sync from one source of truth.

Design notes

Two layers, kept strictly separate. The trap is driving a Spectre
Progress/Live region from inside an ILogEventSink — those are stateful
console-region takeovers and can't be fed by decoupled events. Logging layer
= the event stream; presentation layer = the frame.

Tasks

• SpectreConsoleSink : ILogEventSink — level→colour mapping,
  Markup.Escape on rendered text (paths/arrays/GUIDs contain [ ]),
  AnsiConsole.WriteException for exceptions
• NDJSON sink via Serilog.Formatting.Compact.CompactJsonFormatter (CLEF)
  → stdout
• OutputMode flag + logger build that branches: human → Spectre sink,
  json → CLEF sink (branch, don't run both — avoids ANSI corrupting the
  NDJSON stream by construction)
• stdout/stderr discipline in json mode: NDJSON on stdout, all
  diagnostics/human chatter on stderr
• Promote end-of-run summary to a Spectre Table (executor-driven,
  human mode only)
• JSON-mode structured summary record (single event with results as
  properties) as the equivalent of the table
• Wire into the host output/logger configuration (swap inherited console
  sink)

Acceptance criteria

• fallout build renders rich human output incl. summary table; no markup
  parse errors on real-world messages (paths, arrays)
• fallout build --output json emits valid NDJSON on stdout, one object per
  line, parseable by jq, zero ANSI contamination; diagnostics on stderr
• Exceptions render via Spectre in human mode and are captured (@x) in json mode

Out of scope (follow-ups)

• Progress/Live spinners — human-only polish, separate item; this is where
  the two layers can fight over the console
• Custom semantic NDJSON schema — start on CLEF, diverge only if
  describe --json consumers need a bespoke contract (cost: re-handling all
  property-value unwrapping)

Refs

• Keep sink package surface minimal — assembly-version conflicts in the
  build-loaded context are a known NUKE pain point