Add an incremental build example

[OzzyCzech]

Documents @sphido/cache@1.0.0 the way this repo documents things — as something you can run.

cache/

A fifth example using all three functions: changed() skips pages whose output is up to date, removed() reports sources that disappeared, writeCache() records the state after the build. It prints what it did, so the point is visible:

$ node index.js
3 page(s) built, 0 skipped — run it again and nothing is built

$ node index.js
0 page(s) built, 3 skipped — run it again and nothing is built

Two details the example demonstrates rather than explains:

• the build hashes itself into the version key, so changing the layout rebuilds every page — the cache watches content, not templates
• page.output is set before changed(), which is what makes rm -rf public regenerate the site

content/about.md doubles as the mtime vs hash demo (touch it and the default strategy notices; hash does not), and content/blog/hello-world.md invites you to delete it and watch removed() report it.

Verified by hand

step	result
first run	3 built, 0 skipped
second run	0 built, 3 skipped
edit one page	1 built, 2 skipped
delete a source	gone: content/blog/hello-world.md …, 0 built
rm -rf public	2 built

Also

• cache added to the CI matrix, so it is built on Node 22 and 24 like the others
• a section in the root readme
• cache/.gitignore covers .sphido/ next to public/

Depends on #2 only in spirit — that one unbreaks main, this one is additive.

[OzzyCzech]

CI note: cache on Node 22 and cache on Node 24 pass — this example is fine.

The red jobs (basic, extenders, json, rss) fail on npm install, not on anything here: this branch is off main, which still asks for @sphido/markdown@^1.0.0. #2 fixes that. Merge #2 first and these clear on the next run.

[OzzyCzech]

Carried #2's one-line bump into this branch (9554164), so the PR no longer depends on main being fixed first — every example now asks for @sphido/markdown@^2.0.0 and CI should be green across the matrix.

Both merge orders are safe: if #2 goes in first, this branch's copy of that change becomes a no-op.