[fm-var] Add inspect and explain CLI workflows

[DandyLyons]

Problem / outcome

Expose read-only inspect and explain workflows so users and automation can understand every RFC 001 Rev 3 reference, source, projection, JSONPath result, effective format, cache, and status before validation or synchronization.

Epic: #111. Milestone: 4 — CLI workflows.

In scope

• Add fm-var inspect / explain workflows following existing CLI conventions.
• Support files/content trees, exclusions, hidden-file behavior, and deterministic document ordering through GlobalOptions.
• Inspect element kind/location, raw src/query, resolved source, YAML-to-I-JSON projection metadata, nodelist cardinality/order, selected node/value shape, effective formatting, cached/expected content, and status.
• Explain source policy, query-argument conversion, RFC 9535 query steps/functions, first-node or one-sequence rules, split fallback selection, coercion, cascade, and failures.
• Human-readable and versioned structured JSON output with safe redaction/escaping.

Relevant specification sections

• §2 all Rev 3 attributes.
• §3.4–3.8 resolution, projection/querying, cardinality, fallback, and formatting.
• §4 JSONPath querying/YAML projection.

Dependencies and blockers

• Blocked by [fm-var] Reconcile lossless parser with RFC 001 Rev 3 #118, [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 query evaluation, fallbacks, atomicity, and escaping #120, [fm-var] Implement fm-format cascade and effective configuration #117, [fm-var] Implement ECMA-402-compatible date and time formatting #121, and [fm-var] Implement one-sequence fm-list rendering #119.
• Does not depend on filesystem mutation.

Implementation considerations

• Keep runners/renderers separate from Argument Parser structs for direct Swift Testing.
• Avoid printing credentials, denied contents, full sensitive values, or unbounded nodelists.
• Surface RFC object-enumeration ordering caveats and configured query limits without implying stronger determinism.
• Provide stable reference identifiers based on document/source location.
• Consume provenance emitted by Core rather than recomputing semantics.

Acceptance Criteria

• Users can inspect all references in a file/tree without mutation.
• Explain covers source resolution, query-argument projection, JSONPath/functions, nodelist/cardinality, selection, split fallback, coercion, cascade, expected cache, and status.
• Output includes <fm-format> and both scalar/list references using raw decoded/authored query representations safely.
• JSON output is versioned, deterministic within RFC guarantees, source-located, parseable, and unstyled.
• Denied/sensitive sources and large query results are redacted/bounded appropriately.
• No inspect/explain path writes files.

Test requirements

• Parser/runner/renderer tests, structured-output decoding, multi-file ordering, nodelist/cardinality cases, query limits, selector ambiguity, redaction, and process boundary tests.

Documentation / CLI-help impact

• CLI help, DocC, README examples, and structured report field definitions using Rev 3 terminology.

Explicitly deferred

• Validation exit policy, writes, interactive selection, editor UI, network enablement, and browser hydration.