perf: parallel component builds in wilco build

[hartym]

Summary

build_components() in build.py spawns one esbuild subprocess per component sequentially. With 50+ components, build time grows linearly with no parallelism.

Current behavior

for name, component in registry.components.items():
    result = bundle_component(...)  # sequential subprocess

Proposed solution

Use concurrent.futures.ThreadPoolExecutor (4-8 workers) since Python just waits on esbuild subprocesses. Alternatively, investigate esbuild's multi-entry-point bundling to eliminate per-process spawn overhead.

Also consider incremental builds: compare source file hashes against existing manifest, skip unchanged components.

Context

Found during comprehensive review of feat/pre-compile-bundles-v2 (finding #16).