fix(sync): avoid canonicalize on WASI preopened dirs

[lwshang]

Ports the WASI canonicalize fix from the migration branch (#71) to main.

Problem

The sync plugin's scan step canonicalized the manifest dir. On wasm32-wasip2, Path::canonicalize calls realpath, which returns ENOENT for any path beneath a preopen whose WASI guest name has more than one component (e.g. src/frontend/dist) — even though read_dir/metadata/read through that preopen work fine. So a nested dir failed:

failed to run plugin
  caused by: plugin returned error: canonicalize src/frontend/dist: No such file or directory (os error 44)

Single-component dirs like dist happened to canonicalize to /dist and worked, which is why the existing fixtures never caught it. (Originally reported on the forum against the migration recipe; same root cause lives here.)

Fix

Build the scan root by prepending / to the (host-validated, relative, ..-free) manifest dir instead of canonicalizing. This yields the same shape canonicalize produced for single-component dirs (/dist, /src/frontend/dist) and uses only plain wasi:filesystem ops the preopen supports. main's scan() had a single canonicalize call; walk already uses read_dir paths rooted at that dir, and _headers/_redirects load via relative paths — none of which need canonicalization.

Test

Adds an e2e fixture + nested_dir_deploy test using dirs: [src/frontend/dist] with a nested subdir asset. Verified it fails with the exact error above when the fix is reverted, and passes with it.

• cargo test -p sync-core — 188 pass
• cargo test -p e2e --test sync — 7 pass (incl. nested regression)
• cargo clippy -p sync-core — clean

Note: this is the same fix already shipped on migration as migration-v2.2.1 (consumed by asset-canister-v2.2.1).