fix(state): preserve 'version:' on b.yaml round-trip (--fix, oci:// tags)

[fentas]

Summary

`BinaryList.MarshalYAML` emitted `LocalBinary.Enforced` as YAML `version:` and ignored `LocalBinary.Version` entirely. Two visible consequences:

1. `b install --add oci://docker@cli` produced `oci://docker: {}` in `b.yaml` — the tag (`cli`) was loaded into `Version` (via `version:`) and never written back out.
2. `b install --fix jq@v1.7` wrote `version: v1.7`. On reload only `Version` was populated, so the `--fix` pin was silently lost; next `b update` could move jq off the pinned version.

Fix

Emit `Version` as `version:` and `Enforced` as `enforced:` in `BinaryList.MarshalYAML`. The unmarshal path already maps them to distinct YAML keys, so they now round-trip independently.

Workflow

Split into two commits for clarity:

1. `test(state): failing regression` — adds 4 sub-tests for `MarshalYAML` and a file-round-trip test, all currently failing.
2. `fix(state): preserve 'version:' on b.yaml round-trip` — implements the fix; new tests pass; two existing tests updated from the old buggy semantics to the correct ones.

End-to-end reproduction

Before:
```
$ b install --add oci://docker@cli:/usr/local/bin/docker
$ cat b.yaml
binaries:
oci://docker:/usr/local/bin/docker: {} # ← tag wiped
```
After:
```
binaries:
oci://docker:/usr/local/bin/docker:
version: cli
```

Test plan

• `TestBinaryListMarshalYAML_VersionRoundTrip` (4 sub-tests: version-only, enforced-only, both-set, oci-tag-as-version).
• `TestBinaryListMarshalYAML_VersionSurvivesFileRoundTrip` — Load → Save → reload.
• `go test ./...` — all 39 packages pass.
• Manual e2e: `b install --add oci://docker@cli:/usr/local/bin/docker` now writes `version: cli` to b.yaml.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes b.yaml round-trip behavior for binary configuration so version: and --fix pins persist correctly across Load → Save, including OCI provider tags.

Changes:

• Update BinaryList.MarshalYAML to emit LocalBinary.Version as version: and LocalBinary.Enforced as enforced:.
• Update existing marshal tests to reflect the new distinct YAML keys.
• Add regression tests to ensure version: survives both MarshalYAML and a full file round-trip.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pkg/state/types.go	Adjusts YAML emission so version and enforced are serialized to distinct keys.
pkg/state/types_test.go	Updates expectations and adds regression coverage for version: round-trips and OCI-tag preservation.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.