infra: replace qrcode/png/uri with best-in-class external libraries (optional conan deps)

[fpelliccioni]

Current state

• qrcode: wraps libqrencode (C library, with_qrencode conan option, default off). 92 lines of wrapper code.
• png: with_png conan option exists. Used for QR code output as PNG.
• uri: hand-written RFC 3986 parser (~200 lines). No external dependency.

Goal

Replace these with the best available external libraries, keeping them as optional conan dependencies (same pattern as today). Evaluate:

QR code

• If there's a library that generates QR codes directly as PNG (or SVG), that would eliminate the need for a separate PNG dependency. Candidates:
  • nayuki/QR-Code-generator — header-only C/C++, generates QR matrix (no image output)
  • libqrencode (current) — C library, generates QR matrix, needs separate PNG lib for image
  • Look for a single lib that does QR → PNG in one step

PNG

• Only needed if the QR library doesn't produce PNG directly
• libpng (likely current), stb_image_write (header-only, simpler), etc.

URI

• ada-url — fast, standards-compliant URL parser (used by Node.js). Available on conan.
• Or keep the hand-written parser if it's sufficient (it's small and works)

Conan integration pattern

Follow the existing with_qrencode pattern:

options = {"with_qrencode": [True, False]}
default_options = {"with_qrencode": False}

def requirements(self):
    if self.options.with_qrencode:
        self.requires("libqrencode/4.1.1", ...)

All three should remain optional — the node works without QR/PNG/URI support.