docs: More ADR-0003 updates

[zharinov]

Refine tagged alternation serialization model

Summary

Updates the JSON serialization design for tagged alternations to use a uniform { $tag, $data } structure, avoiding collisions and simplifying implementation.

Changes

• $tag discriminator: Changed from tag to $tag to avoid collisions with user-defined @tag captures
• $data wrapper (always): All payloads wrapped in $data for uniform structure—no conditional flatten-vs-wrap logic
• Removed strict constraint: Tagged branches accept any payload type (objects, arrays, primitives)
• Added alternation guidance: Documented when to use untagged (simplicity) vs tagged (precision)
• Updated docs: README.md, REFERENCE.md, and ADR-0003 reflect the new serialization rules

Serialization format

{ "$tag": "Assign", "$data": { "target": "x", "value": {/* ... */} } }
{ "$tag": "Items", "$data": [1, 2, 3] }
{ "$tag": "Name", "$data": "foo" }

Rationale

• Uniform structure eliminates conditional logic
• Always access via .$data — predictable for codegen and LLMs
• Zero collision risk with user captures
• Mirrors serde's adjacently-tagged representation (#[serde(tag = "$tag", content = "$data")])