Support complex mapping keys

[MariusStorhaug]

Why

YAML allows arbitrary nodes (sequences, mappings) as mapping keys via the ? indicator, but PowerShell hashtables and [PSCustomObject] only accept scalar keys, so this maps awkwardly today and can fail unexpectedly. Complex keys are rare in real-world YAML, making this the lowest-priority spec gap.

What

Parse ? complex keys without erroring, with a clear and predictable resolution strategy for how a complex key is represented in the returned object.

Spec examples covered

• Example 2.11 — Mapping between Sequences

[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.