Resolve historical @version installs from the ledger, not .versions

[kelp]

Problem

FetchRecipeVersion still depends on .versions files for non-head pinned installs (gale install jq@1.8.1), even on current clients:

• internal/registry/registry.go:456–533 — historical versions fall through to the .versions commit-pin path.

Latest-install already resolves from the [[history]] ledger in .binaries.toml; only historical @version pins still require .versions. This is the one genuine code dependency blocking deletion of the .versions bridge in gale-recipes (see kelp/gale-recipes cutover issue).

Proposal

• Resolve @version from [[history]] ledger entries (recipe/binaries.go already has MergeBinariesForRecipe).
• Keep .versions only as a fallback for pre-ledger clients; that fallback disappears with the cutover.

Acceptance

• gale install <name>@<old-version> resolves correctly with no .versions file present.
• Must merge before the gale-recipes .versions cutover PR.

[kelp]

Cutover tracked in kelp/gale-recipes#100 — this client change must merge first.