feat(portable_text): per-subtree config via PortableTextTheme

[Ortes]

What

Adds the ability to render different Portable Text subtrees with different configurations, instead of every widget sharing the single global PortableTextConfig.shared.

This mirrors Flutter's own Theme pattern:

• PortableTextConfig.of(context) — resolves the nearest config from the tree, falling back to shared when none is present (like Theme.of / ThemeData.fallback).
• PortableTextConfig.copyWith(...) — returns a derived config. Map fields (styles, blocks, blockContainers, markDefs) are merged (provided keys win, the rest are kept); scalars are replaced. Deliberately differs from ThemeData.copyWith because these maps are additive registries.
• PortableTextTheme — an InheritedWidget that supplies a config to its subtree.
• PortableTextStyleOverride — ergonomic helper: pass per-style TextStyle → TextStyle deltas (e.g. {'normal': (s) => s.copyWith(fontSize: s.fontSize! - 2)}); it wraps the inherited builders internally (no recursion, no boilerplate).

The render paths (PortableTextBlock, defaultListBuilder, defaultBulletRenderer) now read PortableTextConfig.of(context) instead of .shared directly.

Backward compatibility

Fully non-breaking. With no PortableTextTheme ancestor, of(context) returns shared, so existing apps render identically. The constructor is made public so additional configs can be created.

Note on mark deserialization

_markDefsFromJson runs at JSON-parse time with no BuildContext, so it intentionally keeps reading the context-free shared.markDefs registry. Mark styling is still resolved per subtree at render time. This is documented inline.

Commits

1. feat — the per-subtree config support (the actual change; upstream-ready).
2. chore — drops resolution: workspace so the package resolves standalone when consumed as a git/path dependency outside the vyuh workspace. Fork-only — should not be merged upstream.

[Ortes]

Superseded by the upstream contribution vyuh-tech#57. A fork-internal PR isn't the point — the feature goes upstream. The Fullphysio app consumes the change via a pinned git override on the feat/portable-text-per-subtree-config branch (which additionally carries a fork-only resolution: workspace removal needed for standalone git-dependency resolution) until the upstream PR is merged and published.