feat: own the index and log entry format in one model

[chrishiguto]

Why

Rules, prepared writes, and MCP writes must not carry competing copies of the index.md and log.md entry grammar.

What

Adds OkfIndexDocument and OkfLogDocument as the shared parser/emitter owner. The index generator and fixed Spec rules consume that model while bundles continue to retain reserved files as raw text.

Writable models canonicalize entry text, preserve the intended index order, sort log date groups newest-first, and refuse states that cannot round-trip—including CR/LF-bearing links. Parse results retain malformed input for rule reporting and cannot be converted into writable documents while issues or unsupported frontmatter remain.

Reviewer's guide

Start with lib/src/index_log.dart, then lib/src/index_generator.dart, lib/src/spec_rules/context.dart, and lib/src/spec_rules/reserved_rules.dart. This is cleanly restacked on the merged graph query work.

Testing

• Formatting and fatal-info analysis are clean.
• All 123 tests pass.
• Entry round trips, malformed-input reporting, CR/LF rejection, validator-compatible emission, and byte-identical existing index generation are covered.
• dart pub publish --dry-run reports zero warnings.

Links

Part of #15. BundleChangeSet consumption lands in #27.