Add representation-preserving YAML stream merging

[MariusStorhaug]

Why

Configuration authors need to combine layered YAML streams without losing representation details such as tags, complex keys, anchors, aliases, recursive graphs, or mapping order.

What

Add Merge-Yaml so two or more complete YAML streams can be merged deterministically. Later streams have higher precedence, document counts must match, and the result is one normalized YAML 1.2 stream.

Acceptance criteria

• Accept two or more complete YAML streams through -InputObject [string[]] or the pipeline; each array element or pipeline record is one stream
• Merge documents pairwise by zero-based document index and reject empty streams or differing positive document counts with classified errors
• Merge compatible mappings recursively by YAML structural key equality while retaining base entry order and appending new overlay keys
• Support Replace, Append, and Unique sequence policies; Replace and Error conflict policies; and Replace and Ignore null policies
• Preserve selected representation nodes, standard and unknown tags, complex keys, anchors, aliases, repeated nodes, cycles, and ordinary << mapping data without object projection or YAML 1.1 merge expansion
• Enforce parser, cloning, equality, merge, and result resource budgets
• Emit exactly one deterministic LF-only YAML string with no final newline
• Include complete help, README and example usage, package export and generated documentation coverage, built-artifact tests, and corpus-backed smoke coverage

[MariusStorhaug]

Corrected the work item to the delegated Merge-Yaml scope.nn- Replaced the stale Remove-YamlEntry request with the representation-preserving stream merge contractn- Recorded mapping, sequence, conflict, null, graph, resource-budget, test, documentation, and stacked-delivery decisionsn- Replaced the implementation checklist so issue 43 remains the source of truth for this change

[MariusStorhaug]

Completed the implementation plan and delivery validation.nn- Added the public representation-preserving merge command, graph helpers, documentation, package export, and built-artifact coveragen- Completed the table-driven policy, graph, multi-document, resource-limit, deterministic, and corpus smoke matrixn- Passed source and built-artifact Pester, source and module lint, generated docs/site builds, and Process-PSModule on Windows, Linux, and macOSn- Delivery remains in draft PR #45 targeting format-yaml`

[MariusStorhaug]

What changed: Reformatted the issue body to the MSXOrg Issue Format — a local Why plus an outcome-level What/acceptance criteria. Removed the Technical Decisions and Implementation Plan sections (and other implementation-tradeoff notes).
Why: Issue bodies should state local purpose and observable outcome without leaking implementation detail into the outcome-level description; durable technical decisions belong in the PR/design, not the tracking issue.
Follow-up: None.