fix(text): inherit parent styles in nested Text

[Doberjohn]

Motivation

Two related bugs in nested Text, both reproduced on main first.

A variant parent lost its typography. When a Text with a variant wrapped a
nested Paper Text, it replaced its own font with the child's style instead of
layering on top of it. With an unstyled child there was no child style to take
over, so the parent rendered with no typography at all. This reached shipped
components: Chip renders <Text variant="labelLarge"> around its children, so
<Chip><Text>label</Text></Chip> dropped the entire label font block
(fontSize: 14, fontWeight: 500, lineHeight: 20, letterSpacing: 0.1).

A nested Text did not inherit. A nested Text or AnimatedText without a
variant applied its fallback font unconditionally, overriding the family,
weight, letter spacing and color it should have inherited from the enclosing text.

On the approach: React Native's Text already inherits from an enclosing Text
natively, so nothing needs to merge styles. The bug was Paper overriding what
would otherwise be inherited. A nested component now declares only its own
style. The shared context therefore carries a boolean rather than a style, and
lives in its own module so the Text and AnimatedText imports stay
one-directional.

AnimatedText was broken in both directions and is fixed too. It falls back to
bodyMedium where Text falls back to fonts.default, so an animated child
inside a displayLarge parent was snapping to fontSize: 14. That fallback
inconsistency between the two components still exists, but is no longer
reachable through nesting.

Related issue

Fixes #4351.

#4356 is an earlier attempt at the same issue, taking the opposite approach of
merging the parent's style into the child. It also only publishes the parent
style from the variant-less branch, so a child under a variant parent would
still not inherit, and it is written against a version of Text.tsx that no
longer exists on main.

Test plan

Example app, Typography screen, new "Nested text" section with four cases:
unstyled child, styled child, child with its own variant, and a bold parent
whose nested child inherits the weight.

• 6 new unit tests. Each behaviour-changing one was confirmed to fail without its fix.
• The 3 pre-existing nesting tests still pass, so the intentional
  child-overrides-parent behaviour is preserved.
• AnimatedText had no test coverage before this; it now has 3 tests.
• Full suite: 55 suites, 740 tests, 169 snapshots, with no snapshot churn.
• yarn lint and yarn typecheck clean.

[Doberjohn]

A quick note on the Typography work: because a nested Text without a variant no longer applies fonts.default at all, this fix is immune to Body variants switching typeface. Before it, that switch would have caused a visible sans-serif inside sans-serif-medium mismatch on Android. A nested Text that declares its own variant still applies it, which is unchanged here.