Selectors: JSONPath filters, recursive descent, negative index and multi-value headers #44

Why

body.$.<path> is a gjson subset translated by ToGJSON (internal/selector/selector.go:79-111): dotted paths, [n], ["quoted"], # and *. Real JSONPath users (Hurl, Postman, Bruno) also write filters, recursive descent, negative indexes and slices, and those fail to parse today. Headers only expose the first value.

What

Support, with the same syntax in # @assert, # @capture, the BDD the response body "<path>" steps and MCP:

• body.$..id (recursive descent; gjson has no .., so implement a small in-tree walker for this form and document the choice)
• body.$.items[?(@.done == true)].id and [?(@.name =~ /^a/)] (map onto gjson #(done==true)#.id and #(name%"a*"))
• body.$.items[-1], body.$.items[1:3]
• .length as an alias of # for people arriving from JavaScript
• header.<name>.# (count) and header.<name>[1] (n-th value) for multi-valued headers such as Set-Cookie and Link

Errors name the unsupported construct and the line. validate uses the same parser so a bad selector is caught before running.

Where

internal/selector/selector.go, selector_test.go; internal/bdd/steps.go (paths without $.).

Docs

docs/format.md (Selectors table), docs/cheatsheet.md, docs/testing.md (body paths paragraph).

Tests

Table-driven ToGJSON cases with the expected gjson path and the expected value on a fixture document; multi-value header cases; parse-error messages.