Improve default `dataTooltip` with richer information from adapted payloads

[dlrice]

Context
When a track has no kind, no dataTooltip, and no tooltipOverrides entry, the resolver synthesizes a minimal fields spec from the common feature-shaped record (type, description, start/begin, end) — see src/tooltips/resolve.ts. This is a reasonable floor, but it throws away a lot of information that adapters actually produce into the payload: scores, cross-references, sequence context, consequence types (for variants), AlphaMissense pathogenicity bands, and so on. Authors who haven't yet written a custom dataTooltip see a visibly impoverished tooltip compared to the old hard-coded ones, even when the adapter has clearly populated richer fields.

Goal: make the default tooltip pull more from what's already on the payload (including fields emitted by calculate transform steps) so that an out-of-the-box config produces a useful tooltip without the author writing one.

Task
Expand the default tooltip synthesizer in src/tooltips/resolve.ts to surface more of the adapted payload automatically, and render it via the Markdoc path so formatting (links, emphasis) matches custom tooltips.

Scope:

• Extend the fallback field list beyond type/description/start/end to include, where present: score (formatted per-kind), consequenceType, clinicalSignificances, xrefs (as linkified names where a source→URL map exists), evidences (count + first source), and any fields added by calculate steps.
• Emit the fallback through the Markdoc renderer rather than a flat <h5>/<p> grid, so that (a) links become anchors with the same URL-scheme allowlist as author-written tooltips, and (b) the visual treatment matches when authors progressively customize.
• Preserve the existing precedence: any tooltipContent the adapter stashed on the item still wins; any author-written dataTooltip still wins over the default.
• Document the expanded fallback set in specs/config-approach.md under the tooltip section (currently ~L869 — "Sensible out-of-the-box fallback"). List the fields surfaced, in what order, and with what formatting.
• Add a snapshot/DOM test covering: variant record → expected default tooltip, feature record with xrefs → expected default tooltip, minimal feature record → same minimal tooltip as today (no regression).

Notes:
Keep the fallback compact — this is a safety net, not a replacement for authored tooltips. If the payload has fifteen fields, surface at most ~six; the rest are the author's call to include with an explicit dataTooltip. Pick the subset based on which UniProt/AlphaFold/variant adapters actually populate the fields — don't infer from type heuristics alone.