[fm-var] Add end-to-end conformance coverage and documentation

[DandyLyons]

Problem / outcome

Turn the completed RFC 001 Rev 3 vertical slices into a coherent interoperable feature with end-to-end conformance coverage, examples, documentation, and discoverable CLI help.

Epic: #111. Milestone: 5 — Completion.

In scope

• End-to-end tests spanning parse → resolve → YAML/I-JSON projection → RFC 9535 query → cardinality/selection → coerce → cascade/format → validate/preview/write.
• Interoperability checks against language-neutral fixtures and accepted Rev 3 examples.
• README, architecture, common use cases, DocC, CLI help, security/limits, diagnostics/exits, and Rev 2-to-Rev 3 migration examples.
• Update/synchronize canonical and bundled Agent Skill files.
• Validate Apple/Linux/Core WebAssembly boundaries and reproducible locale guidance.

Relevant specification sections

• Entire RFC 001 Rev 3, especially §§3–5 and §9.

Dependencies and blockers

• Blocked by [fm-var] Add preview and explicit-write synchronization #113 and all Milestones 1–4 prerequisites.

Implementation considerations

• Prefer black-box workflow fixtures for public behavior and focused unit tests for boundaries.
• Keep locale-dependent strings separate from portable query/projection semantics.
• Follow Swift Testing and store isolated fixture copies under project tmp/.
• Keep both bundled Skill files byte-identical.

Acceptance Criteria

• Standard document and representative cross-file workflows pass end to end using Rev 3 query syntax.
• Inspect, explain, validate, preview, and write examples are covered at appropriate CLI boundaries.
• Fixtures cover RFC selectors/functions, YAML projection failures, node order/duplicates/cardinality, scalar/list success, zero/null fallbacks, stale/denied/malformed/coercion/format/resource failures, escaping, atomicity, Unicode, and LF/CRLF.
• README/DocC/architecture/common-use-cases/help/security/diagnostic/exit/migration docs are complete and consistent.
• Canonical and bundled Agent Skill documents are synchronized.
• swift build, swift test, native Linux checks, and Core WebAssembly validation pass or limitations are documented.
• Locale/ICU and RFC object-order variability plus reproducible-CI expectations are documented.

Test requirements

• Golden end-to-end fixtures, process boundary tests, structured-output decoding, help assertions, query-limit security cases, Linux and WebAssembly checks.

Documentation / CLI-help impact

• Own the final public documentation/help pass, Rev 2 migration guidance, and cross-linking to the authoritative Rev 3 specification.

Explicitly deferred

• Browser hydration, editor integrations, unaccepted remote/query capabilities, general templating, and post-v1 syntax.