perf: add content-hash caching to skip generation when inputs unchanged

Hashes the drizzle-zero package version, config file content, tsconfig
content, and CLI flags into a SHA-256 cache key. On cache hit, returns
the previously generated output immediately, skipping ts-morph project
creation and schema generation entirely.

Cache file (.drizzle-zero-cache) is stored next to the output file.
Atomic writes (write to tmp, rename) prevent corruption. A --no-cache
flag is available to force regeneration.

Author: Alp