fix(build): reuse primary assets in worktrees [AGENT]

[utensil]

Summary

Let a clean Jujutsu worktree reuse browser assets from the primary checkout.

• Copy a missing WASM package only when its recorded source hash and required files match.
• Use the primary checkout's theme asset when the worktree lacks the submodule.
• Keep CI and UTS_DEV source builds unchanged; fail early when no valid local cache exists.

Scale

Changes build.sh and justfile by +51/-12 lines.

Verification

• A fresh worktree restored all three package trees exactly and passed just build.
• A second build passed from the worktree copies.
• An invalid primary root failed before any source clone.
• ShellCheck reports no new finding in the changed lines.

Human-gated. Auto-merge is disabled.

(per G-scope, G-lint, G-verify, G-commit, G-safe)