feat(cross): vendor cargo modules and prefetch pub for offline builds

[jwinarske]

Extends the acquisition step so cargo and pub inputs are materialized too — an offline module build now has every crate and package on disk. Builds on the offline fence from #98.

What

• Cargo vendoring. emb fetch (and emb cross --fetch-only) vendor each build: cargo module into the shared store with cargo vendor --locked, content-addressed by the module's Cargo.lock. The lock is found by walking up to the enclosing repository (stopping at .git), so a crate that locks at a Cargo workspace root is not mistaken for unpinned.
• Offline consumption. The offline build points CARGO_HOME at the vendored config, so cargo --offline resolves crates from the on-disk vendor dir instead of the registry.
• Pub prefetch. emb fetch --app <dir> runs flutter pub get --enforce-lockfile to populate PUB_CACHE and the app's .dart_tool for the offline build.
• Native targets. emb fetch now serves local/host too — no toolchain to resolve, but it still vendors crates and prefetches pub.

Structure

• New lib/src/cross/cargo_vendor.dart: cargoLockDir (repo-boundary walk-up), vendorConfig (absolute-path rewrite), the CargoVendor class (vendor/locate), and vendorTargetCargo/hasCargoModules shared by emb fetch and emb cross --fetch-only.

Test

• dart analyze lib test clean; dart format --set-exit-if-changed clean.
• New cargo_vendor_test.dart (walk-up incl. the .git boundary, config rewrite, vendor with a fake runner, locate); extended fetch_command_test.dart (--app guard, native path).
• Full suite: 498 pass, 1 skip, and the same 2 pre-existing real dpkg-deb/real debugfs tests (fail on main too).

Docs

README updated in this PR: the emb fetch section (new --app row, vendoring/prefetch prose) and the Offline builds subsection.

Follow-ups

• Store-rooted PUB_CACHE (this prefetches into the ambient cache the build inherits) and pub prefetch parity on emb cross --fetch-only.
• The netns enforcement ladder + --offline=strict (denies emb's own egress today; does not yet sandbox a subprocess).