[fm-var] Implement one-sequence fm-list rendering

[DandyLyons]

Problem / outcome

Implement RFC 001 Rev 3 <fm-list> evaluation/rendering for a JSONPath query that selects exactly one one-dimensional scalar YAML sequence, including split empty/null fallbacks and whole-list atomic failure.

Epic: #111. Milestone: 3 — Formatting and lists.

In scope

• Require a nodelist containing exactly one sequence node; $.steps is valid while $.steps[*] selects members and is a cardinality/shape error.
• Extract only the selected sequence's immediate members, preserving order and duplicates without recursive flattening.
• Apply default-zero to zero selected nodes or an empty selected sequence.
• Apply default-null to a selected null node or a sequence containing only null members.
• Reject multiple selected nodes, scalar/mapping selections, mixed null/scalar sequences, nested sequences/mappings, and any invalid member atomically.
• Apply item-type coercion to every valid scalar member.
• Render ordered/unordered success caches as exact list HTML and inline formats through locale-aware list formatting.
• Render block or inline fallbacks as escaped literal text with no list/list-item wrapper.
• Compare existing caches for freshness without trusting child markup.

Relevant specification sections

• §2.2 <fm-list> Rev 3 attributes, placement, formats, styles, and fallback cache shape.
• §2.3 array defaults/cascade.
• §3.6–3.9 cardinality, shape, split fallback, formatting, and escaping.
• §8 list-result rationale.

Dependencies and blockers

• Blocked by [fm-var] Reconcile lossless parser with RFC 001 Rev 3 #118, [fm-var] Implement RFC 9535 JSONPath querying #124, [fm-var] Resolve RFC 3986 sources and project YAML query arguments #112, [fm-var] Define source-access and query-resource policy #114, [fm-var] Implement scalar coercion and default serialization #126, [fm-var] Implement scalar query evaluation, fallbacks, atomicity, and escaping #120, and [fm-var] Implement fm-format cascade and effective configuration #117.
• Inline list parity depends on the closest available Foundation/ICU list formatter.

Implementation considerations

• Reuse scalar coercion per item but stage every result before emitting an edit.
• Successful block serialization permits only required list/list-item markup; fallback serialization uses scalar-cache text escaping.
• Keep locale-sensitive adapters isolated from portable cardinality/shape/fallback semantics.
• Record query nodelist cardinality, selected sequence metadata, effective locale/style, and failure member location for explain output.

Acceptance Criteria

• Exactly one selected sequence succeeds; zero, null, multi-node, and wrong-shape outcomes follow their distinct Rev 3 rules.
• Empty sequences use default-zero/unresolved-zero; all-null sequences use default-null/unresolved-null.
• Mixed-null, nested, mapping, or invalid members retain the complete prior cache with stable diagnostics.
• Sequence order and duplicate members are preserved.
• Ordered/unordered successes emit only canonical HTML; block fallbacks emit only escaped literal text.
• Inline formats/styles and item-type behave consistently and atomically.
• Member/fallback text cannot inject HTML/Markdown or close an fm-var element.

Test requirements

• Language-neutral fixtures for $.items versus $.items[*], nodelist cardinality, empty/all-null/mixed-null, shape/coercion, duplicates/order, split fallbacks, atomic failure, escaping, and block structure.
• Fixed-locale tests for list types/styles, one/two/many items, Unicode, and Apple/Linux behavior.

Documentation / CLI-help impact

• Examples for query-to-sequence semantics, both fallbacks, all five formats, three inline styles, locale requirements, and block placement/fallback shape.

Explicitly deferred

• Rendering arbitrary JSONPath nodelists directly, recursive flattening, mappings, custom item templates, Markdown list-marker caches, and arbitrary child markup.