refactor: consolidate host/authority/path serialization

[OmarAlJarrah]

What

Host / authority / URL-path serialization was reimplemented in several places instead of delegating to the canonical renderers.

• parser/Resolver held a byte-for-byte copy of host/HostSerialization's serializeHost (the host when, the zone-id suffix, the "%25" constant) — a latent drift bug: change host serialization and the resolution path silently diverges.
• Url.encodedPath and Url.authority re-implemented Serializer's URL-path and authority serialization, including the non-obvious leading-/. path guard.
• An internal Host.serialize() extension duplicated the public Host.asText() (identical body).

Change

• Resolver now calls serializeHost; its copy is deleted.
• Promoted Serializer's path/authority helpers to internal top-level serializeUrlPath / serializeAuthority; Url delegates to them and Serializer keeps using them.
• Removed the Host.serialize() synonym and routed callers to asText(). This drops one tautological Ipv6Test that only asserted serialize() == asText(); asText() remains covered by the host-validation / ipv4 / ipv6 suites and the Kover floor still holds.

Left duplicated on purpose

• Uri.reconstructAuthority — its nullable-userInfo empty→null distinction can't be expressed by the string-only shared helper.
• Resolver's authority assembly — the logic is identical to the shared helper, but importing serialize into parser would form a package cycle with the existing serialize→parser edge (UriNormalizer→Resolver.removeDotSegments). Four trivial lines aren't worth a cycle.

Safety

Pure refactor — no behavior change, public API unchanged (apiCheck passes with no apiDump). Verified on the JVM gate + koverVerify + a Kotlin/Native macosArm64 compile. Net −53 lines.

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Stacked on #46.