hashavatar 0.13.0

0.13.0 expands the background catalog and applies targeted determinism hardening without adding new avatar families.

Added

Added seven AvatarBackground values:
- polka-dot
- striped
- checkerboard
- grid
- sunrise
- ocean
- starry
Implemented each new background in raster and SVG output.
Added tests that prove the new raster backgrounds are distinct and that the new SVG backgrounds remain well-formed XML.

Raster frame-shape hit-testing now uses integer arithmetic for circle, squircle, hexagon, and octagon masks. This reduces one source of platform-specific floating-point rounding in clipped output.
Automatic style derivation can now map some identities to different backgrounds because AvatarBackground::ALL contains more variants. Services that need old automatic output should keep their existing namespace style_version until they intentionally migrate.
Documentation now avoids cargo test --all-features guidance because blake3 and xxh3 are intentionally mutually exclusive crate-wide identity hash modes.
README fuzz wording now reflects default WebP coverage and feature-gated encoder paths instead of describing PNG as always available.

Explicit AvatarOptions and AvatarStyleOptions selections remain deterministic for the selected kind, background, dimensions, seed, namespace, crate identity hash mode, and identity.
The new background variants are public API and therefore affect automatic background distribution.
No new avatar families were added in this release.