json module: expand with path access and helpers via vendored cJSON

[SchoolyB]

Context

The json module currently exposes only encode, decode, pretty_print, is_valid. That is barely enough to be useful — users cannot pull a nested value out of a decoded object without verbose manual traversal.

A dedicated JsonValue tagged union was considered and rejected for now. Stay with generic maps.

Approach

Vendor cJSON (small, permissive, C) under ezc/vendor/cjson/. Same pattern as sqlite.

Additions

• get(obj, "path.to.key") — dotted path accessor, returns value or null
• has(obj, "path.to.key") — existence check
• set(obj, "path.to.key", value) — set value at path, creating intermediate objects as needed
• remove(obj, "path.to.key") — delete a key at path
• keys(obj) — top-level keys
• type_of(value) — return "null" / "bool" / "number" / "string" / "array" / "object"

Out of scope

• Streaming decode
• Schema validation
• Typed unmarshaling into structs (revisit in 3.1)

Target

3.0 GA