humanspeak / svelte-markdown / 25831598991

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fix: streaming nested HTML + agent-output positioning (#292)

* test(perf-bench): close measurement gaps identified during #286

Adds the metrics and scenarios that were missing from the harness when
investigating the render-dominates-parse problem in #286 — specifically
the signals you'd want before opening that issue, not after.

- Parser instance counter: dev-only window hook in Parser.svelte
  (gated on import.meta.env.DEV so production bundles drop it). The
  perf-bench page resets it per scenario and exposes parserInstances
  in the stats line. This was the non-wall-clock signal that cracked
  open the original investigation; future regressions in component
  allocation count will now show up directly.
- renderOnlyMs as a first-class column: firstPaintMs - parseColdMs.
  Surfaces parse-side wins that pure firstPaint hides when render
  dominates, instead of forcing manual subtraction off the side.
- Per-scenario observer snapshots: scenarioLongestTaskMs,
  scenarioMutations, scenarioLoafScriptMaxMs are filtered to the
  scenario's [start, end] window rather than the rolling-10s
  aggregate, so cross-scenario correlation isn't needed to attribute
  longtasks or mutation churn.
- parse-realistic scenario: a mixed-shape corpus generator that
  varies paragraph length, alternates ordered/unordered lists, mixes
  code-block languages, includes occasional tables/blockquotes/inline
  HTML. Defeats optimizations that only win on the highly-repetitive
  generateLarge shape.
- parse-50kb-overridden scenario: same body as parse-50kb but the
  live preview mounts with custom paragraph + text + html_div snippet
  overrides. Drives the slow dispatch path (the one users actually
  hit when they customize), so override-path regressions can't slip
  past the bench.
- stream-bursty scenario: jittered chunk sizes (1-10 typical, rare
  30-50 bursts) and inter-chunk delays drawn from short/medium/long
  buckets, pacing seeded with mulberry32 for reproducibility. Add... (continued)

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