feat(state): add context.select for granular page-scoped reads

[jacobaraujo7]

What

Adds context.select<T, R>(selector) to ModularStateX — the method-based twin of the existing Selector widget. It mirrors context.select from provider to make migration a near drop-in.

// Inside build(): rebuilds only when cart.items.length changes.
final count = context.select<CartViewModel, int>((cart) => cart.items.length);

The calling widget rebuilds only when the selected value changes (==), instead of on every notification (which is what context.watch does).

Why

We already ship Consumer/Selector (provider-style widgets). context.select rounds out the migration surface so projects coming from provider can move method-by-method without restructuring their widget trees.

How

A plain InheritedNotifier notifies every dependent on each trigger, so it can't do per-widget selection on its own. _VMInherited now uses a custom InheritedElement that:

• reproduces InheritedNotifier's trigger-driven rebuild behavior, and
• honors per-dependent selector aspects — on each notification it re-evaluates each selecting dependent's selector and only rebuilds it when the derived value changed.

A watch dependent (no aspect) is still notified on every trigger, so watch/read/Consumer/Selector behavior is unchanged.

Tests

• New test/select_test.dart: rebuilds only on selected change, ignores unrelated changes, keeps tracking across rebuilds, and throws when no scoped T is provided.
• Full suite green (56 tests), flutter analyze clean.

Notes

• Bumps flutter_modular to 7.0.2 (README + CHANGELOG + v7 state-management docs updated).
• Docs MCP re-embedded and bumped to 0.2.2 (tool/docs_mcp), binary recompiled & smoke-tested locally.

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