feat(build): add BuildValue for values computed once per build

[Princesseuh]

Adds BuildValue: a value computed once per build from content and shared across every page that reads it via ctx.build_value(&MY_VALUE).

static ARTICLE_COUNT: BuildValue<usize> =
    BuildValue::new(|ctx| ctx.content::<Article>("articles").entries().count());

// in a routes render():
let count = ctx.build_value(&ARTICLE_COUNT);

This replaces the pattern of one route computing something in its render() and stashing it in a global for other routes to read. On incremental builds the producing route can be served from cache — skipping its render() — which leaves the global unset and breaks its readers. A build value is computed lazily on read, memoized for the rest of the build, and records its own content dependencies onto every page that reads it, so readers re-render exactly when the values inputs change.

Also removes the unused assets() method from the ContentContext trait, and PageContext::from_static_route / from_dynamic_route now take a &BuildValueStore — both only affect code that drives the build loop directly instead of going through coronate().

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