docs(readme): lead with Trace; add data-quality + retrieval upgrade paths

Restructures the SDK README as a three-act story centred on the product
Trace delivers, not the product internals.

Act 1 -- Trace. 20-second onboarding: install + API key + three
copy-paste-runnable blocks (RAG groundedness, code-agent phantom
scoring with session chaining, stateless session rollup). Each block
shows the fields that matter for routing: score, band,
context_coverage_ratio, context_unused_ratio, session_signals
.recommendation, noise_pct, retrieval_waste_pct, recommendations.

The Act 1 "signals" table makes the story causal -- which Trace output
triggers which downstream action -- rather than marketing.

Act 2 -- Upgrade data quality. Condensed pipelines + DataPackage +
Dataset Intelligence. Triggered by low context_coverage_ratio / amber
bands. Points deep content to SDK_TUTORIAL and docs/pipelines.md.

Act 3 -- Upgrade retrieval. Triggered by high context_unused_ratio /
retrieval_waste_pct. Links to ColSearch (voyager-index,
https://github.com/ddickmann/colsearch) as the drop-in retrieval
upgrade.

docs/trace.md polish: adds a "Which lane should I use?" intro bullet
block; adds the exact client-side ValueError strings under each method;
fixes the primary_metric response row from the input-side Literal to
str | None with a note that the pod echoes derived names like
groundedness_v2.

docs/README.md: promotes Trace to the top of the Direct Service APIs
table so the doc hub matches the README story.

100% SDK compliance gate run before commit:
- every identifier resolves to a real export in src/latence/__init__.py
- every kwarg in trace.{rag,code,rollup}(...) examples is a real
  parameter in src/latence/resources/trace.py
- every response attribute (.score, .band, .context_coverage_ratio,
  .context_unused_ratio, .session_signals.recommendation,
  .next_session_state, .noise_pct, .retrieval_waste_pct,
  .recommendations, .reason_code_histogram, .risk_band_trail) exists
  on the real model in src/latence/_models/trace.py
- all 15 python blocks parse; trace + refinery tests green (86/86)

Author: latenceainew