[cargo-verus] feat: `cargo verus new` should use the auto-detected `vstd` version

[mkovaxx]

Currently the vstd version written into the created Cargo.toml is hardwired, and updated by the bump_crate_versions tool (run in the Crate Updates CI flow: crate-updates.yml).

Once the Verus Toolchains information encapsulated in cargo-verus is ready, it will be able to detect the correct vstd version (based on the verus version) and inject that into the created Cargo.toml.

The critical gap at the moment: the Release CI flow (release.yml) must first be updated to create and commit a new file in the cargo-verus/toolchain-manifests directory. Without that, the TOOLCHAINS information bundled into a cargo-verus release is currently empty/stale. See also:

verus/source/cargo-verus/toolchain-manifests/0.2026.06.07.cd03505.toml

Lines 1 to 7 in d73ef89

	# Not a real toolchain manifest.
	# Only here temporarily to test that the `build.rs` works in CI.
	# It should be removed when the `release.yml` CI flow is extended.
	verus = "0.2026.06.07.cd03505"
	vstd = "0.0.0-2026-05-31-0205"
	z3 = "4.12.5"
	singular = "4.3.2"

For more details on the Verus Toolchains design, refer to this document: LINK.