fix(docs): merge batch assets in combine job to fix client-side routing 404

[ronaldtse]

Problem

Clicking a formula on /browse/ changed the URL but showed 404. Refreshing the page loaded it correctly. Classic client-side routing failure.

Root cause

VitePress produces different content-hashed JS chunks per build when the page set differs. Each build-batch job runs its own vitepress build, so 9 batches produce 9 distinct app.HASH.js files. The combine job was deploying dist-main's assets/ directory only — but browse HTML from each batch references batch-specific hashes that weren't in the deployed assets.

Verified by downloading artifacts from the latest deploy (run 27699130632):

Artifact	app.HASH.js hash
dist-main	CUFvjMGd
dist-batch-0	tggRrD32
dist-batch-1	lhlnUi-u
dist-batch-2	wADyZrW9
dist-batch-3	NxYOTzkh
dist-batch-4	TU-EUwTC
dist-batch-5	j-JEY3xz
dist-batch-6	Bh3mvEjZ
dist-batch-7	Sac3zhiF
dist-batch-8	BldWXoD6

None matched. Browse HTML from batch-N references batch-N's hash, which wasn't deployed → client-side navigation 404s on the missing chunk. The SSR HTML was correct (which is why refresh worked, masking the bug).

Fix

In docs.yml's combine job, also cp -r each batch's assets/ into dist/assets/. Content-hashing guarantees no filename conflicts (same content → same filename, different content → different filename). Total added payload is ~9 small duplicate-ish app.HASH.js chunks (~few KB each), acceptable for a static site.

Verification

After merge and deploy:

1. Go to https://www.fontist.org/formulas/browse/
2. Click any formula — should load directly (no 404, no refresh needed)
3. Confirm in browser DevTools that /formulas/assets/app.<HASH>.js returns 200 (was 404)

Why this was masked

• The PR fix(docs): scope link_checker to real links, fix broken SPDX URLs #263 SSR HTML is correct, so manual URL navigation or refresh works
• Client-side routing failure only manifests on in-page click navigation
• The previous "click → 404" reports were attributed to the clean URLs issue, which PR fix(docs): scope link_checker to real links, fix broken SPDX URLs #263 fixed
• This is a separate, deeper issue that PR fix(docs): scope link_checker to real links, fix broken SPDX URLs #263's verification missed (because Playwright test navigated directly via URL, not by clicking links)