feat(driver): add getBuildCode() to extract current build as shareable code

[SFerenczy]

Summary

Adds getBuildCode() as the symmetric counterpart to the existing loadBuildFromCode(). Serializes the current build back to the standard PoB share-code format — base64url(zlib(xml)) — so embedders can round-trip: load a code → user/agent edits → read the code back out.

The shape mirrors loadBuildFromCode exactly; changes are threaded through all four layers:

Layer	Addition
packages/driver/boot.lua	getBuildCode() Lua function — calls build:SaveDB("code"), deflates, URL-safe base64
packages/driver/src/c/driver.c	get_build_code() exported C function with static buffer lifecycle matching the existing s_* pattern
packages/driver/src/js/worker.ts	getBuildCode() on DriverWorker + cwrap binding
packages/driver/src/js/driver.ts	getBuildCode(): Promise<string> public API on Driver

Motivation

I'm embedding pob-web in a build-analysis app where an AI agent mutates the build XML. After mutation, I need to serialize the current in-memory build back to a code to persist/share it. Today loadBuildFromCode gets you in to pob-web; there's no symmetric way to get out. This closes that gap with a minimally-invasive addition.

Design notes

• No new dependencies — reuses common.base64 and Deflate already in PoB, same as loadBuildFromCode.
• Error surface matches existing code — Lua-side error() on missing mainObject.main / wrong mode, C-side prints to stderr and returns NULL on pcall failure, matches the style of load_build_from_code in the same file.
• Static buffer s_build_code follows the existing pattern in driver.c; freed on next call.
• URL-safe base64 (+→-, /→_) matches what loadBuildFromCode accepts, so the output is a drop-in input for the inverse call.

Test plan

• Manual round-trip: await driver.loadBuildFromCode(code); const out = await driver.getBuildCode(); and confirm the resulting XML (after base64/inflate) equals the input XML for at least one non-trivial build
• Error path: call getBuildCode() before a build is loaded — expect a rejected promise with a clear error
• Verify dist/driver.wasm size doesn't meaningfully regress

Happy to adjust naming, error handling, or scope if you'd prefer a different shape.