mcp: add detectForms tool for structured form discovery

Add a detectForms MCP tool and lp.detectForms CDP command that return
structured form metadata from the current page. Each form includes its
action URL, HTTP method, and fields with names, types, required status,
values, select options, and backendNodeIds for use with the fill tool.

This lets AI agents discover and fill forms in a single step instead of
calling interactiveElements, filtering for form fields, and guessing
which fields belong to which form.

New files:
- src/browser/forms.zig: FormInfo/FormField structs, collectForms()

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lightpanda.zig

@@ -35,6 +35,7 @@ pub const markdown = @import("browser/markdown.zig");
 pub const SemanticTree = @import("SemanticTree.zig");
 pub const CDPNode = @import("cdp/Node.zig");
 pub const interactive = @import("browser/interactive.zig");
+pub const forms = @import("browser/forms.zig");
 pub const actions = @import("browser/actions.zig");
 pub const structured_data = @import("browser/structured_data.zig");
 pub const mcp = @import("mcp.zig");