Make the API queryable: ship api.json, add cui api / cui icons, document the traps #7

Context

Building a dashboard meant repeatedly grepping skills/cui-dotnet/api/*.md for struct fields (ChartOpts, Series, TableColumn, EmptyOpts, MetricOpts, SegmentedOpts). The generated truth already exists in the repo (bindings/api.json), but it is neither in the skill nor queryable from the CLI.

Icons are worse: Icons is generated with no index in the docs, so RefreshCw was guessed as REFRESH_CW and ChartLine as CHART_LINE — both compile errors.

Two undocumented facts also changed the code I wrote:

• Size has implicit operator Size(float) (src/CuiSharp/Interop/Types.cs:101). The docs only show Size.Px(...), so I wrote Size.Px(92) everywhere.
• ChartOpts.Labels is matched to points by index, not spread over the axis. Verified: 5 labels on 100 points renders only the first ("Open") and silently drops the rest, while 100 labels are auto-thinned when painted. Likewise Series.Count == 0 means "draw the whole array", so a partially filled Series[] is a trap.

Proposal

• Ship api.json (or a slimmed variant) inside the skill/package, and add cui api ChartOpts / cui api LineChart to print the signature and fields.
• Add cui icons [filter] to list generated icon names.
• Add a one-page "signatures & traps" index covering implicit conversions, Labels/Count semantics, and the Size.Fill + chart Height rule.
• Keep the generated pages as the long tail; make the index the entry point.

Acceptance

Any type, member, or icon name is resolvable with a single CLI call; no markdown search required.