refactor(zig): drop deprecated 0.16 APIs and tighten io/allocator threading

[forketyfork]

Issue

Complete the requested Zig 0.16 migration cleanup. No issue number was supplied for this maintenance change.

Solution

• Replace deprecated bufPrintZ calls with bufPrintSentinel.
• Use init.gpa in the runtime, manage URL launches through a bounded joinable opener, and simplify spawnDetached.
• Route shell forks through the errno-aware POSIX wrapper and use std.testing.io in the listed tests.
• Update the Zig 0.16 API notes and architecture documentation.

Context

The runtime receives its allocator from main. The URL opener owns joinable worker threads, copies the caller-provided allocator and I/O context into each worker, and joins active workers during runtime shutdown before startup teardown. No new dependencies were added.

Test plan

• On macOS with a display, click an OSC 8 terminal URL and verify it opens in the browser.
• On macOS with a display, open the config file from the menu and verify it still opens in the configured editor/browser path.