refactor(v8-bridge): split the bridge bundle into a documented multi-file TypeScript package

[NathanFlurry]

• Split the single 28k-line crates/execution/assets/v8-bridge.source.js bundle into a multi-file TypeScript source tree under packages/build-tools/bridge-src/ (prelude, polyfills, global-exposure, transport, vendor/{esbuild-runtime,buffer}, and builtins/{active-handles, buffer-constants, fs, os, child-process, network, whatwg-url, events, process, module-loader}).
• Add a build:bridge-source step that bundles the TypeScript modules back into the readable v8-bridge.source.js seam, keeping node/bare/undici-shim imports external so the downstream minify build is unchanged.
• Keep v8-bridge.source.js as a generated seam so all existing text-consumers (contract validation, polyfill-registry check, util-polyfill codegen, limits audit) keep working without changes.
• Wire the generated seam into the generated-artifacts check so drift is caught in CI.
• Preserve the sidecar/js/executor interface: no changes to the bridge contract, installed globals, module surface, or dispatch behavior.
• Fix a pre-existing browser bridge-contract drift (bench metrics globals) and refresh the stale generated WASI polyfill surfaced while wiring the check.