feat(resolvespec): add 'contains' operator for array overlap

* Implemented 'contains' operator using BuildArrayOverlapCondition for real array containment.
* Updated documentation to clarify differences between text-cast ILIKE and array overlap.

pkg/restheadspec/HEADERS.md

@@ -88,7 +88,7 @@ This will match any records where the column contains the search term (case-inse
 Search with specific operators (AND logic).
 
 **Supported Operators:**
-- `contains` - Contains substring (case-insensitive)
+- `contains` - Contains substring (case-insensitive). Implemented as `CAST(col AS TEXT) ILIKE '%value%'` for every column type, including arrays (stringifies the array, then substring-matches). **Not** array containment — no GIN index use, and can false-positive on partial matches within array elements. resolvespec (a different spec package in this repo) defines `contains` differently: real PostgreSQL array-overlap (`&&`). Don't assume the two behave the same.
 - `beginswith` / `startswith` - Starts with (case-insensitive)
 - `endswith` - Ends with (case-insensitive)
 - `equals` / `eq` - Exact match

pkg/restheadspec/README.md

@@ -96,6 +96,8 @@ X-Limit: 50
 
 **Available Operators**: `eq`, `neq`, `gt`, `gte`, `lt`, `lte`, `contains`, `startswith`, `endswith`, `between`, `betweeninclusive`, `in`, `empty`, `notempty`
 
+> Note: `contains` here is a text-cast ILIKE substring match (works on any column type, including arrays, by stringifying first) — not array containment. resolvespec's `contains` operator has different semantics (real array overlap). See [HEADERS.md](HEADERS.md) for details.
+
 For complete header documentation, see [HEADERS.md](HEADERS.md).
 
 ## Lifecycle Hooks