Add Optional Parley Text Engine

[CryZe]

This adds Parley as an optional path based text engine alongside the
existing cosmic-text implementation. It can be selected with the
parley-text-engine feature, including together with
software-rendering or svg-rendering, while cosmic-text remains the
default for existing users.

Parley handles font matching, fallback, shaping, bidirectional text, and
glyph positioning, while Skrifa extracts the resulting glyph outlines.
The implementation preserves the existing renderer-facing font and
label API, including ellipsis truncation, tabular number features,
monospaced digit emulation, variable font coordinates, and glyph path
caching.

This also supports COLR version 0 color fonts by expanding CPAL-colored
layers into the ordered paths understood by the software and SVG
renderers. Rendering tests have separate expected hashes for Parley
where its shaping, fallback, or path generation differs from
cosmic-text.

Draft Status

This is currently a draft because Parley / Fontique has a known system
font fallback problem on Windows, tracked in
linebender/parley#644.

Fontique currently asks DirectWrite for a single fallback family for a
script and locale by probing a representative script sample. For Han,
the sample is 今, which causes DirectWrite to select Yu Gothic UI on
the affected system. Fontique then caches that as the only Han fallback
family. Yu Gothic UI contains 字 and 漢, but not simplified 汉,
even though another installed family such as Microsoft YaHei UI
contains all three. As a result, Parley renders 汉 as the missing-glyph
symbol.

The implementation remains opt-in until fallback selection can consider
the actual text or expose enough ordered fallback families for Parley's
coverage checks to select a suitable font.

Validation

• cargo check --no-default-features --features parley-text-engine
• cargo test --no-default-features --features std,software-rendering,parley-text-engine --test rendering
• cargo test --no-default-features --features std,svg-rendering,parley-text-engine --test rendering
• cargo check --all-features
• cargo clippy --no-default-features --features std,localization,software-rendering,parley-text-engine --lib -- -D warnings