Reconcile Inlay Hints (F23) into canonical design docs

[clrudolphi]

Inlay hints for feature files (F23) shipped a while ago (#43, with follow-up fixes #57 and #77), but the as-built reality was never folded back into the canonical docs/LSP-IDE-Support-* design docs, and docs/InlayHints-Implementation-Plan.md's status header is stale (still says "Draft for review" despite the feature being fully implemented: GherkinInlayHintService, FeatureInlayHintHandler, InlayHintRefreshHandler all exist and are wired up in Program.cs). See docs/AsBuilt-Reconciliation-Reminder.md.

Work

• Update docs/InlayHints-Implementation-Plan.md's status header to reflect it shipped.
• LSP-IDE-Support-Feature-Designs.md: add a new feature § for Inlay hints — final label format (lift the plan's §11 visual spec + the shipped mockup image), settings defaults, resolve-vs-eager split as built.
• LSP-IDE-Support-Architecture.md: register the new components (GherkinInlayHintService, FeatureInlayHintHandler, InlayHintRefreshHandler).
• LSP-IDE-Support-Open-Questions.md: promote the plan's IH-* risk ids into the Q-register, or close them with the as-built answer.
• Fold the plan's "Risks & open questions" into the design doc's deferred/known-limitations section.

Reference

• docs/InlayHints-Implementation-Plan.md
• Add inlay hints showing step binding info (F23) #43, InlayHintRefreshHandler never actually delivers workspace/inlayHint/refresh to the client #57, Fix inlayHint/foldingRange race by declaring capabilities statically #77