ci: optimize build pipeline with caching and parallelization

[bartlomieju]

Summary

Optimizes the CI build pipeline from ~7 min to ~2.5 min (warm cache) or ~5 min (cold cache).

What changed

1. Cache reference types + docs (reference_gen/types/ + reference_gen/gen/)

• On cache hit, skips type extraction (86s) and doc generation (162s) entirely
• Keyed on hashFiles('reference_gen/**') — only regenerates when scripts, configs, or dependency versions change
• First run after merge seeds the cache; all subsequent PRs hit it

2. Cache std-docs (runtime/reference/std/)

• Skips 43 sequential JSR API fetches
• Keyed on hashFiles('scripts/generate_std_docs.ts')

3. Split Lume into two builds

• New _config-reference.ts: minimal config that renders only API reference pages (JSX only — no markdown-it, esbuild, tailwind, sitemap, toc, or OG images)
• CI runs reference build first, then main build with SKIP_REFERENCE=1
• Both write to _site/ with emptyDest: false
• Main build no longer renders hundreds of API pages through the full plugin chain

4. Eliminate redundant reference generation

• Build step runs deno task lume directly instead of deno task build which re-ran generate:reference

5. Parallelize I/O

• afterBuild tasks (lint rule copy, LLM files, md source copy) run concurrently via Promise.all
• Std-docs JSR fetches batched 10 at a time instead of 43 sequential

6. Fix SKIP_REFERENCE check

• Use === "1" instead of env.has(), which incorrectly triggered on empty strings

7. Add build profiling (LUME_PROFILE=1)

• Instruments build phases and individual processor timings
• LUME_PROFILE=1 deno task build:light to use locally

Expected CI times

Step	Before	After (warm cache)	After (cold cache)
Setup + fmt + test	20s	20s	20s
Reference types + docs	248s	~2s	248s
Std-docs	15s	~1s	~5s
Build: reference pages	(in main build)	~30-40s	~30-40s
Build: main site	158s	~80s	~80s
Server + link checker	35s	35s	35s
Total	~7 min	~2.5 min	~5 min

New tasks

• deno task lume:reference — run the reference-only Lume build
• deno task build:split — full build using the split strategy