test: add property-based tests (fast-check) for the pure input/encoding layer

[goanpeca]

Our input and encoding helpers are validated with hand-picked examples. Property-based testing asserts invariants over thousands of generated inputs and is where encoding, boundary, and platform (posix vs Windows) bugs hide.

Proposal: add fast-check as a dev dependency plus a property-test module, kept isomorphic so it runs under Node, Bun, and the browser matrix like the rest of the suite, with a fixed seed for reproducibility.

Candidate invariants, each backed by a real SDK function:

• Filename wire-encoding round-trips: for any string, decodeFileName(encodeFileName(s)) === s, and the encoded form never contains a raw character B2 forbids.
• SHA-1 streaming equals one-shot: for any buffer split at random chunk boundaries, feeding the chunks through IncrementalSha1 equals sha1Hex over the whole buffer.
• Retry backoff stays in bounds: for any attempt index and RetryOptions, the delay is non-negative, grows as documented, and never exceeds the configured cap (jitter included).
• classifyError totality: for any {status, code} it returns a B2Error, preserves .status/.code, and .retryable matches isTransient(status, code).
• SSE-C keys: any 32-byte key round-trips through EncryptionKey.fromBytes (correct MD5, redaction holds); non-32-byte inputs reject deterministically.
• Sync key/path mapping stays within the destination prefix and never emits // or escapes the prefix.

Acceptance criteria:

• fast-check added; a property test module covering the invariants above.
• Runs in the existing Node + Bun + browser flows with a fixed seed and a documented way to reproduce a failing case.

Related: the adversarial/fuzz issue shares the fast-check dependency but targets no-crash/no-injection on hostile inputs rather than invariants.