fix: tolerate multi-document YAML and multiple << merge keys

[FIERsity]

What

Real GitLab configs exposed three parsing gaps that made the gitlab backend hard-error on them:

1. Multi-document YAML (gitlab-org/cli): spec: + pipeline documents; GitLab merges documents in order, later keys win. serde_yaml::from_str rejects the file outright.
2. Multiple << merge keys in one mapping (fdroid/fdroidserver): YAML 1.1 allows it; serde_yaml rejects the duplicate key.
3. !reference custom tags: unknown tags surface as enum variants.

How

New src/yamlx.rs — shared tolerant loader: parses every document (folding duplicate << keys into a sequence, preserving order), merges documents recursively, resolves merge keys with YAML 1.1 semantics (explicit keys > earlier merges > later merges), and preserves unknown-tag values. All four backends (Compose/Actions/GitLab/CircleCI) parse through it.

Bonus: GitLab top-level release: is a job name, not a keyword — it was silently dropped; removed from the non-job list.

Verification

• Both real configs that previously errored now scan cleanly (glab: 9 jobs incl. multi-doc globals; fdroid: merge anchors resolved)
• 2 new unit fixtures from the real files + 9 yamlx unit tests (incl. !reference)
• 132 tests green, clippy/fmt clean
• validate-real-repos.sh extended with GitLab-hosted coverage

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Review details

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Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 08567810-5741-4f43-989b-b7e7f90b2c64

📥 Commits

Reviewing files that changed from the base of the PR and between f823cf7 and 67d586e.

📒 Files selected for processing (12)

• CHANGELOG.md
• README.md
• scripts/validate-real-repos.sh
• src/actions.rs
• src/circleci.rs
• src/compose.rs
• src/gitlab.rs
• src/lib.rs
• src/yamlx.rs
• tests/cli.rs
• tests/fixtures/gitlab/double-merge.yml
• tests/fixtures/gitlab/multi-doc.yml

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