refactor(hm-vm): Magic "ephemeral" sentinel string passed through snapshot() label couples vm.rs and docker.rs

[markovejnovic]

Smell

Vm::snapshot took a &str label that carried two orthogonal meanings plus an implicit grammar:

• In vm.rs, when the caching policy was CachingPolicy::None, the label was the literal magic string "ephemeral"; otherwise it was the cache key.
• In docker.rs, snapshot() re-parsed that string as a mini-language: splitn(2, ':') to extract repo:tag, treating a bare label (the "ephemeral" case) specially by tagging the commit with the container id.

So a single &str encoded (a) cache-key-vs-ephemeral-marker and (b) an implicit repo:tag grammar. The producer (vm.rs) and consumer (docker.rs) had to agree on this convention out-of-band, and the "ephemeral" literal was duplicated across files where it could silently drift.

Change

Introduce a typed SnapshotLabel enum in hm-vm:

pub enum SnapshotLabel {
    Ephemeral,
    Cached(String),
}

• Vm::snapshot now takes &SnapshotLabel instead of &str.
• vm.rs maps CachingPolicy::None -> SnapshotLabel::Ephemeral and CachingPolicy::Cache { key } -> SnapshotLabel::Cached(key.clone()) — the "ephemeral" magic string is gone from the producer.
• The Docker backend matches on the variant: Ephemeral commits under the container-id tag; Cached(key) parses repo:tag from the key. The magic literal and the splitn convention become a single typed contract.

Behavior is preserved exactly: the ephemeral path still tags with the container id, and the cached path still splits on the first : (split_once(':') matches the old splitn(2, ':') semantics, including bare keys falling back to the container-id tag).

Type-safety pattern

Applies fd's "Token/Template enums for a mini-language instead of raw strings" pattern, and the general "parse once into a typed value" doctrine: the ephemeral-vs-cached distinction is now a compiler-checked enum rather than an implicit naming convention shared across two files.

CI / validation note

Only cargo check -p hm-vm (incl. --tests) was run locally, per the scope of this change; it passes. Full CI runs on this PR. The transformation is mechanical and behavior-preserving, but this is a draft pending review.