fix(fs-node): infer promises.readFile results from encoding

[Godsdar]

fs.promises.readFile('/foo', 'utf8') currently returns Promise<string | Buffer> in TypeScript, forcing callers to cast before using string methods.

Add encoding-sensitive overloads to both exported FsPromisesApi interfaces and share that signature with the concrete class. Text encodings infer Promise<string>; omitted encoding and memfs's 'buffer' encoding infer Promise<Buffer>. Dynamic strings/options retain the union. Emitted JavaScript is unchanged.

Add regression tests for encoding strings/objects, flags and binary results. Before the fix, these fail with six TS2322 assignment errors.

Fixes #702.

Validated locally on macOS arm64, Node 26.9.0 / Yarn 4.12.0:

• yarn build, yarn typecheck, yarn prettier:check: passed.
• yarn test: 100 suites passed; 1,619 tests passed, 18 skipped; 14 snapshots passed.
• git diff --check: passed.
• Targeted TSLint: three pre-existing failures in FsPromises.ts — duplicate imports at line 7 and two Function type bans at line 21. Baseline and patched lint results are identical; no new lint failures.

Scope is promise API readFile inference; FileHandle.readFile and other method overloads are separate work. No dependency changes. The full upstream OS/Node CI matrix has not been run locally.

AI assistance was used for investigation, implementation, testing and agent code review.