feat(data-contracts): make postcard a first-class JSON-free Linkable codec

[thaodt]

Description

• Split the format-neutral Linkable contract from its JSON convenience.
• Make generated Postcard codecs an exercised, JSON-free embedded path.
• Add mixed JSON + postcard compile coverage to prevent codegen dependency drift.
• Extend the repository validation matrix and update affected documentation.

Context

After #170, Linkable already provides the typed byte-oriented boundary requested by #155. Adding another generic Codec<T> in aimdb-core would duplicate that boundary.

This PR therefore implements the re-scoped #155: harden postcard as the compact binary codec for shared-contract links while keeping self-describing remote interfaces unchanged.

Compatibility

This changes the semantics of the existing linkable feature. JSON derive users must migrate from:

features = ["linkable"]

to:

features = ["linkable-json"]

Manual Linkable implementations and postcard/custom codecs should continue using the base linkable feature.

Because changing linkable requires JSON derive users to opt into linkable-json, this PR also bumps aimdb-data-contracts from 0.3.0 to 0.4.0.
The version requirements in aimdb-sync, aimdb-wasm-adapter and aimdb-websocket-connector are updated accordingly.

Out of Scope

• A new genericCodec<T> abstraction in aimdb-core
• Changes to JsonCodec, AimX, MCP, CLI or [FEAT] Evaluate CBOR as the self-describing IR in the type-erased core (alt to serde_json::Value) #156
• Making postcard the global default
• Zero-allocation encode_into(&mut [u8])
• Selecting a different codec for each link of the same record type

Related Issue

• Closes [FEAT] Make postcard a first-class, JSON-free Linkable codec #155

Checklist

• I have read the CONTRIBUTING.md document.
• My code follows the project's coding standards.
• I have added tests to cover my changes.
• All new and existing tests passed (make check).
• I have updated the documentation accordingly.

[lxsaah]

lgtm!

[lxsaah]

@thaodt I filed #177 and #178 as follow-up. Please tell me what you think. Thanks again!