view: route paths are not URL-encoded, and a malformed % hangs the page #2071

Found in the review of #2057, which adds the experimental view. It does not block that PR; #2059 fixed the findings that did. Paths refer to #2059's head.

Problem

parseRoute and formatRoute in web/view/src/route.ts:

• The path is neither encoded nor decoded. A path with a space or non-ASCII characters cannot be opened, and an address the page wrote with replaceState fails after a reload.
• The ref is decoded with decodeURIComponent, which throws URIError on #o/r@%/a.gcl. main.ts parses the route at module top level, so the page stays at "loading…".
• The Route type allows { ref: null, base: "x" }, which means nothing.
• Repository names starting with . (such as .github) are rejected, as are GitHub URLs with a line anchor (#L10).

Proposed fix

• Encode and decode each path segment, and check for ., .. and empty segments after decoding.
• Return null when decoding fails.
• Make Route a discriminated union.