fix: HashHistory Link hrefs escape the hash router

[jligeza]

Problem

With HashHistory, <Link> renders an anchor whose href is the plain route-space path (e.g. /posts/123) instead of the hash form (#/posts/123).

Plain left-clicks still work because Link's onClick intercepts them and routes through router.navigate, which goes through HashHistory.push (the only place that prepends #). But native browser link actions — Ctrl/Cmd-click, middle-click, "copy link address", "open in new tab" — use the href directly and bypass the click handler. On a static host (e.g. GitHub/GitLab Pages) the browser then requests /posts/123 from the server, which doesn't exist, so the link is broken.

The root cause is that the mapping from an internal URL to a real browser URL lives only inside each history's push. Link has no read-only way to ask the history "what href should this URL render as", so it falls back to the raw path.

Fix

Add an optional createHref(url) to HistoryLike:

• BrowserHistory / MemoryHistory: identity (url => url)
• HashHistory`: `url => `#${url}`

Link now sets href: router.history.createHref?.(url) ?? url.

• Navigation is unchanged — navigate still pushes the plain url, and HashHistory.push still adds the single #, so there is no double-#.
• createHref is optional on the interface, so existing custom HistoryLike implementations keep working (they fall back to identity).
• BrowserHistory/MemoryHistory hrefs are byte-for-byte unchanged.

Verification

Rendering a real <Link to="/posts/123"> through a real HashHistory router with react-dom/server:

Before: <a href="/posts/123">
After: <a href="#/posts/123">

BrowserHistory.createHref('/posts/123') → /posts/123 (unchanged), HashHistory.createHref('/posts/123') → #/posts/123.