[fm-var] Implement fm-format cascade and effective configuration

[DandyLyons]

Problem / outcome

Implement <fm-format> validation and the specification-defined cascade so each scalar/list reference has an explainable effective formatting configuration.

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

In scope

• Parse and validate document defaults, for token sets, compatible attributes, duplicates, and placement.
• Apply precedence: spec defaults; later compatible unscoped declarations; later matching scoped declarations; local element attributes.
• Support targets string, boolean, integer, number, date, datetime, timestamp, and array.
• Retain provenance for every effective option so explain output can identify its winning declaration.
• Diagnose unknown/incompatible attributes and invalid declarations without silently applying them.

Relevant specification sections

• §2.1/§2.2 local formatting attributes.
• §2.3 <fm-format> placement, targeting, compatibility, and precedence.
• §3.8 supported formatting option names.

Dependencies and blockers

• Blocked by [fm-var] Reconcile conformance fixtures and core models with RFC 001 Rev 3 #123 (models) and [fm-var] Reconcile lossless parser with RFC 001 Rev 3 #118 (lossless parsing).
• Concrete consumers are [fm-var] Implement ECMA-402-compatible date and time formatting #121 (date/time) and [fm-var] Implement one-sequence fm-list rendering #119 (lists).

Implementation considerations

• Represent a typed option registry in MarkdownUtilitiesCore rather than scattered string switches.
• Separate raw declaration diagnostics from cascade calculation; an invalid declaration must have defined effects on otherwise valid declarations/references.
• Preserve declaration source order and location from the parser.
• Make effective configuration/provenance Codable for structured explain output.

Acceptance Criteria

• All allowed for tokens and compatible v1 options are modeled.
• Cascade precedence and later-declaration wins behavior match the spec.
• Local attributes override document defaults.
• Unscoped declarations apply each attribute only to compatible references.
• Incompatible/unknown attributes, invalid for values, and misplaced declarations receive stable diagnostics.
• Effective values include provenance and deterministic ordering for explain output.
• Multiple <fm-format> declarations do not modify source.

Test requirements

• Matrix tests for scoped/unscoped/local precedence, multiple declarations, mixed compatible attributes, every target type, invalid tokens/options, and declaration placement.
• Language-neutral effective-configuration fixtures.

Documentation / CLI-help impact

• DocC cascade table and provenance examples; explain/inspect help later references it.

Explicitly deferred

• Actual locale algorithms, list rendering, mutation of formatting declarations, and editor assistance.