docs: add Column Codecs page

[arv]

What

Adds a new Column Codecs documentation page (contents/docs/codecs.mdx) and a nav entry under Schema.

The page documents the .codec() schema builder API from rocicorp/mono#6087:

• The .codec() API — storage type vs app type, decode (read) / encode (write + filter), and that the type parameter is the app-side type.
• Custom codec examples: a JSON column backed by a class instance, and using Zod 4.1+ z.codec() directly.
• Behavior across query results (auto-decoded), filtering (where encodes app values), ordering (sorts on raw storage values), and mutations (encode on write).

Why

.codec() is a new schema feature with no docs yet. This gives users a single reference for how stored vs app types work and how codecs interact with reads, writes, filters, and ordering.

Notes for reviewers

• Verified every API claim against the implementation PR, the published @rocicorp/zero types, and Zod 4.4.3's type defs (the z.codec() object really does expose synchronous .decode() / .encode() methods that match Zero's expected shape).
• Uses useQuery for both React and Solid, consistent with the rest of the docs (createQuery is deprecated in the Solid bindings).
• Notes that null/undefined bypass the codec, so codec functions only handle real values and compose with .optional().
• The page leans on Temporal.Instant as the running example; Temporal still needs a polyfill in most current runtimes. Left as-is for now — flagging in case we'd rather lead with a plain Date codec.

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Project	Deployment	Actions	Updated (UTC)
zero-docs	Ready	Preview, Comment	Jun 2, 2026 2:03pm