Fix fixed-runtime portable packaging

[jamditis]

What changed

• Remove the leading ./ from the fixed WebView2 runtime path so Tauri packages the runtime directly under its configured directory.
• Use Tauri 2.10's empty INSTALLWEBVIEW2MODE value for fixed-runtime-only NSIS behavior.
• Keep the release verifier strict: msedgewebview2.exe must be directly inside the configured runtime directory.
• Cover forced portable mode, superseded-runtime cleanup, and both Windows 10 AppContainer ACLs with regression tests.

Root cause

Release run 30699825159 successfully built, signed, and installed the fixed-runtime artifact, then found the runtime nested as Runtime/Runtime/.... AudioBud's base resources are a map. Tauri 2.10 adds the configured fixed-runtime path to that map and normalizes the walked source; the previous ./ prefix prevented strip_prefix from matching, so the full source directory was appended below the destination. Tauri 2.10 also renders FixedRuntime as an empty NSIS mode, so the prior fixedRuntime guards never compiled.

This is a follow-up to #39.

Validation

• bun test (285 passing)
• bun run lint
• bunx tsc --noEmit
• bun run build
• Prettier and Rust formatting checks
• git diff --check
• GPT-5.5/low review: no findings
• GPT-5.6 Sol/xhigh review against exact Tauri 2.10 source: no findings