Add optional per-paragraph direction from assembled spans

[st0ck]

Problem and proposal

Related to #65. A single inherited Directionality cannot give English and RTL prose paragraphs independent base directions. This draft adds optional paragraphDirectionBuilder: TextDirection? Function(InlineSpan) to Markdown/MarkdownBody and the builder. It receives the renderer's assembled spans; a non-null result supplies Directionality around the existing paragraph widgets. Null/omitted keeps the existing tree and behavior. No automatic language detection, new dependency or renderer framework.

The hook covers ordinary prose and tight/loose list/blockquote paragraphs, in selectable and non-selectable modes. Formatting and link recognizers remain attached to the same rendered spans. Images/custom widgets are not inspected or rebuilt; their surrounding text segments share paragraph direction. Headings, fenced code, table cells and list markers are excluded. README documents the application-supplied direction policy.

Related work: #147 exposes a general block-widget wrapper for anchors. This proposal is narrower and provides assembled span input rather than AST text or widget-tree inspection. Happy to align the API shape with that work; this draft does not duplicate its wrapper or change its branch. This supplies an opt-in building block for #65, not a claim to implement all automatic RTL behavior.

Callback updates

Changing paragraphDirectionBuilder on an existing Markdown/MarkdownBody triggers the existing reparse path. The callback participates in didUpdateWidget's current invalidation condition; no new cache or key workaround.

Local validation

• Focused paragraph/link/context-menu/selection lane: 168 passed, 3 pre-existing upstream skips.
• Includes 64 paragraph tests: both paragraph orders/UI directions, nested blocks, inherited/null behavior, span identity, formatting, real link taps in both selection modes, excluded blocks, image-separated text and physical start alignment.
• Eight same-mounted-widget cases hold data/style/key/environment and State identity stable while adding/switching/removing the callback or replacing it with one returning null. Actual RenderParagraph/RenderEditable directions and exactly one link dispatch after each update are asserted. All eight failed on stale direction before the lifecycle correction.
• Focused analyzers: no issues. Formatting and whitespace checked.

Test SDK: Flutter 3.41.9 / Dart 3.11.5. Fresh upstream's dev-only mockito ^5.7.0 cannot resolve against that SDK's pinned meta1.17.0, so local tests used an uncommitted pubspec_overrides.yaml with mockito5.6.4. No override, lockfile, SDK or dependency changes are included in this PR. No full CI claim.