describe/title metadata for toJsonSchema

[cryo2010]

Summary

Adds the last item from the JSON Schema polish list: describe(text) and title(text), chainable type-preserving modifiers that attach metadata emitted by toJsonSchema and ignored by everything else:

let user = schema:
  name: string.min(2).describe("Display name")
  role: string.oneOf(["admin", "user"]).title("Role").describe("Access level")

let userDoc = user.title("User").describe("A registered account")

Semantics:

• Invisible to validation: parsing, issues, Infer, toJson, and re-validation are unaffected; the cost is one pass-through node.
• Chained calls merge into a single wrapper, last value wins per field; title and describe coexist.
• A titled recursive schema names its $defs entry (#/$defs/TreeNode instead of #/$defs/def0), with a uniqueness fallback.
• The hardcoded timestamp() description becomes overridable.
• Motivating use case documented in the README: schemas with descriptions are directly usable as LLM tool definitions, which consume exactly this format.

Implementation

One transparent nkMeta node plus pass-through arms in validate/normalize/denormalize and a merge arm in nodeToSchema. The subtle part was peeling the wrapper everywhere node kinds are inspected, each now metadata-tolerant and covered by a test:

• fieldDefOf: alias works in either order relative to metadata
• strict(): peels, applies strictness, re-wraps (metadata survives)
• primKind: integer().describe(...).coerce still passes the coerce guard
• isOptionalField: a described optional field stays out of required
• objFields: pick/omit/partial/merge see fields through the wrapper

Tests

9 new tests (244 total, passing under orc and arc locally). README (modifiers table, JSON Schema section with an LLM tool-definition example, snippets compile-verified) and DESIGN.md updated.