Prepare Wouter 4: modern browser baseline and stronger types

[molefrog]

Wouter 4 raises the compatibility baseline to remove obsolete fallbacks, reduce package size, and improve TypeScript inference. This PR is based on v3.11.0 and keeps its React 18 memory-routing fixes and regression tests.

Compatibility and runtime

• Require React 18.2+ and TypeScript 5.2+; retain Preact 10.x support.
• Target ES2022 with native URL, History, and event APIs. Keep the README requirement concise, with browser coverage estimates and the feature audit in specs/browser-support.md.
• Remove the React external-store shim and legacy hook/browser fallbacks. Retain the snapshot-getter workaround needed by React 18 and the lightweight Preact adapter.
• Use Object.hasOwn() to fix optional-parameter caching when parameter names change while their values remain undefined.

TypeScript

Improve route-literal inference, optional and repeated parameters, typed memory navigation state, search setters, custom parsers, refs, and public hook declarations. Keep the type assertions in the existing package test/*.test-d.ts(x) layout and add strict consumer checks for bundler, NodeNext, and legacy Node resolution.

Validation

• 251 runtime tests pass with 100% source line coverage, including React and Preact SSR.
• Repository type checks, strict consumer checks, lint, and all size limits pass.
• CI covers React 18.2.0, 18.3.1, and 19.0.0, plus TypeScript 5.2.2/workspace and React 18/19 declarations across three resolution modes.
• Compared with v3.11.0 using Bun 1.4.0 and identical dependency versions, the dependency-inclusive React core decreases from 2,641 to 2,388 bytes gzip (9.6%); the Preact core decreases from 2,510 to 2,482 bytes (1.1%). Framework code is excluded.

Both packages are versioned 4.0.0-next.0. Migration notes and reproducible size measurements are included in the compatibility audit.

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size-limit report 📦

Path	Size
packages/wouter/src/index.js	2.08 KB (-8.98% 🔽)
packages/wouter/src/use-browser-location.js	346 B (-37.66% 🔽)
packages/wouter/src/memory-location.js	520 B (-28.67% 🔽)
packages/wouter/src/use-hash-location.js	486 B (-32.5% 🔽)
packages/wouter-preact/src/index.js	2.17 KB (-0.14% 🔽)
packages/wouter-preact/src/use-browser-location.js	462 B (-5.33% 🔽)
packages/wouter-preact/src/use-hash-location.js	605 B (-6.21% 🔽)
packages/wouter-preact/src/memory-location.js	634 B (-4.52% 🔽)