Reuse Ruby builds when static gem extensions are unchanged

[ledsun]

rake npm:ruby-head-wasm-wasi currently forces a Ruby rebuild on every run, repeating linking and Asyncify even when the statically linked gem extensions have not changed.

Add an ExtensionCache that uses the existing extension cache_key methods and stores the input hash beside the Ruby archive. Reuse the archive when the hash matches, and remove the unconditional --remake from npm packaging. Explicit --remake, --reconfigure, and --clean still force a build.

Invalidate the previous record before building, and save a new record only after a successful build whose extension inputs stayed unchanged. Existing archives with extensions but no record are rebuilt once; archives without extra extensions retain their existing reuse behavior. Ruby source, compiler, and build-setting changes still require a manual rebuild.

Validation

• Extension cache tests: 8 tests, 13 assertions passed.
• bundle exec steep check: passed.
• Concrete ruby-head-wasm-wasi package built successfully. With BINARYEN_CORES=1, a rebuilding run took 3m52.19s and a repeated run took 46.78s; both Ruby packaging invocations reused the archive on the repeated run.
• Generated Wasm unit tests: both runs passed 68 tests and 371 assertions.
• Vitest: 44 tests across 7 files passed.
• Browser E2E was not run.