feat: add render_partial for HTMX fragment responses

[papilip]

Summary

Add a render_partial method to Marten::Handlers::Base that renders a template without the layout, context producers, or before_render callbacks.

Motivation

HTMX is increasingly popular for building interactive web applications. A common pattern is returning HTML fragments from handlers to replace parts of the page:

def post
  # Update something...
  render_partial("shared/_item_card.html", context: {item: item})
end

Currently, developers must work around this by manually loading and rendering templates:

tpl = Marten.templates.get_template("shared/_item_card.html")
ctx = Marten::Template::Context{"item" => item}
Marten::HTTP::Response.new(tpl.render(ctx), content_type: "text/html", status: 200)

This is verbose, error-prone, and bypasses the handler's response infrastructure.

Changes

• src/marten/handlers/base.cr: New render_partial method that creates a clean Context (no producers), renders the template, and returns an HTTP::Response
• spec/marten/handlers/base_spec.cr: 3 tests (hash context, named tuple context, custom status)

Design decisions

• No context producers: Partials are fragments — they should not include layout-level data
• No before_render callbacks: These are designed for full-page renders
• No request in context: Keeps the partial isolated. If needed, the caller can add it explicitly
• Supports Hash, NamedTuple, Context, and nil

Related

• Issue Add a Component template tag #165 (Component template tag) — render_partial addresses the server-side use case

Test plan

• Renders template with hash context
• Renders template with named tuple context
• Custom status code works
• All 71 existing handler base tests still pass