Add Position::Static (new default) and Position::Fixed

[nicoburns]

Objective

First step towards browser-correct out-of-flow (absolute/fixed) hoisting: make the Position enum reflect the full CSS position model.

• Position gains Static and Fixed variants, plus is_out_of_flow() / is_positioned() helpers
• Breaking: Position::Static replaces Position::Relative as the default (matching CSS). Static is normal flow but ignores inset and (once hoisting lands) will not act as a containing block for absolute descendants
• Fixed is treated as out-of-flow everywhere Absolute was; it currently behaves identically to Absolute (positioned relative to its parent). True containing-block hoisting (nearest positioned ancestor for absolute, root for fixed) lands in a follow-up PR
• CSS parser (parse feature) accepts all four keywords: static | relative | absolute | fixed
• CHANGELOG.md updated

Context

Part of the OOF-hoisting plan discussed with @nicoburns: candidate bubbling through LayoutOutput, a shared CB-side positioning pass, CB-relative Layout.location, and rounding-via-CB recursion follow in the next PR. This PR only introduces the style-level model so the layout changes can build on it.

Mechanical changes:

• Algorithm checks generalized from position == Absolute to position.is_out_of_flow(), and relative-inset application gated on position == Relative (so Static ignores insets)
• gentest's default serialized position changed from relative to static; test fixtures that relied on the old default for inset behavior now declare position: relative explicitly, and generated tests were regenerated
• Benchmark helpers map Static → legacy Relative and Fixed → legacy Absolute for the taffy 0.3 / yoga comparisons (yoga's PositionType::Static is used directly)

cargo test --workspace, cargo fmt, cargo clippy all pass.

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