[lifetruthuu]

dv: add --rust-target for CPU target selection

--targets selects packages; the CPU architecture was hard-coded to
x86_64-unknown-none in VERIFICATION_RUST_TARGET. This adds a
--rust-target <TRIPLE> option and threads it through every place the constant
was used, so a later architecture can be verified without changing dv again.

Changes

• verus.rs: replace the constant with DEFAULT_RUST_TARGET (x86) and a
  parse_rust_target validator. Introduce SUPPORTED_RUST_TARGETS (accepted)
  and VALIDATED_RUST_TARGETS (x86 only). Experimental targets are accepted
  with a warning that the build is expected to fail.
• verus.rs: pass the triple to cargo-verus (--target), dependency
  resolution, and the cached dependency file. The cache is named
  {package}.{triple}.{release|debug}.deps.toml, so externs for one target or
  profile are never reused for another.
• doc.rs: use the triple for rustdoc --target and for locating artifacts
  under the triple-prefixed directories.
• count.rs: use the triple to find cargo-verus dep-info. count --module now
  selects the #[path] declaration whose cfg(target_arch) matches the target;
  previously it always picked the first declaration, the x86 arch module.
• main.rs: --rust-target on verify, focus, build, doc, and count,
  defaulting to x86. Unsupported triples are rejected at parse time.

Behavior is unchanged for the default target: every command without the flag
builds and verifies exactly as before.

Compatibility

• No change to the existing --targets / --target (doc package) meanings.
• --disasm still reads library_path(), which ignores the triple. The flag
  is currently unused (disassemble() has no caller), so this is not reachable.
  Tracked for when disassembly is wired up.

Verification

• cargo test --lib --bin rust-dv: 11 + 6 passed. Four new tests cover the
  default target, rejection of an unsupported triple, cargo --target
  propagation, and count --module target selection. The count test was
  checked by mutation: disabling the cfg match makes it fail.
• rustfmt --check on the changed files: clean.
• rust-dv count --module arch::irq --rust-target <triple> on the real ostd
  tree resolves x86, riscv, and loongarch to their own irq.rs.
• rust-dv build --rust-target riscv64imac-unknown-none-elf and the
  loongarch64 triple parse and reach the ostd build. Both fail inside ostd
  because of architecture code that is out of scope for this PR; they are
  reported as experimental.

Known test issues, both present on the base commit and unrelated to this PR:

• cargo test --test helper_tests: test_get_dependency fails with
  No targets found (it runs from the dv directory, which has no vostd
  workspace). Not fixed here.
• test_new_package writes verification/test-package and inserts a
  workspace = ... line at the top of dv/Cargo.toml. Run it only in a clean
  checkout, or revert those two changes afterwards. It was skipped for the
  verification above.

Related issues: none.