Add VBlank interrupt lesson to Unbricked

[Peter1169]

Summary

• add a Part II lesson introducing the VBlank interrupt
• add a new unbricked/vblank-interrupt snapshot based on the Objects lesson
• update the later Unbricked examples to wait on a VBlank interrupt flag instead of polling rLY in the main loop
• keep the serial-link example compatible by letting serial interrupts wake halt while only continuing on VBlank

Fixes #157

Tests

• Built all Unbricked ROM snapshots with RGBDS v1.0.0 and rgbfix:
  audio, bcd, bricks, collision, functions, getting-started, input, objects, serial-link, title-screen, vblank-interrupt
• git diff --check
• verified the new mdBook include anchors referenced by src/part2/vblank-interrupt.md
• MDBOOK_BUILD__CREATE_MISSING=false MDBOOK_OUTPUT__LINKCHECK__COMMAND=mdbook-linkcheck MDBOOK_OUTPUT__LINKCHECK__OPTIONAL=false mdbook build
• MDBOOK_BOOK__LANGUAGE=it mdbook build -d book/it

Tooling used for the mdBook checks: mdBook 0.4.52, mdbook-linkcheck 0.7.7, Rust/cargo 1.87.0.