view: check the browser engine's file limit before downloading a project #2072

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

web/view/src/repoProject.ts plans which files to download from GitHub. The browser engine accepts at most 16 files (MAX_PLAYGROUND_FILES in crates/graphcal-wasm/src/project.rs), but the page does not check the count before downloading.

• It downloads every .gcl file under the project root, imported or not.
• Without a manifest, the opened file's directory is the root. tests/fixtures/valid/ in this repository holds 185 .gcl files, including subdirectories. Without a token, that exhausts GitHub's limit of 60 requests an hour.
• With a token, everything is downloaded, then the engine rejects the project.
• For the total path length, Rust counts the entry's path and the page does not.

Proposed fix

• Check the file count, manifest included, before downloading.
• Export the limits from the Wasm engine so they are defined once.
• Download only the package's source directory and the entry file's directory.