feat(config): add remote config layer fetched from S3

[zizou0x]

Stacked on #320 (config centralization).

What

Adds a remote config layer to the resolution stack. Per-chain tuned values are
pulled from S3 at startup and applied right above the embedded defaults:

CLI flags > local config file (fynd.toml) > remote config (S3) > embedded default

Default URL (per chain, overridable via --remote-config-url / REMOTE_CONFIG_URL;
--no-remote-config disables):

https://s3.eu-central-1.amazonaws.com/repo.propellerheads-propellerheads/fynd/presets/{chain}/latest.toml

The payload is a PartialConfig in the same schema as fynd.toml. Empty payloads
are published for all six supported chains (verified end to end against the live
bucket, including a tuned-value round trip).

Fail-safe behavior

The remote layer can never take a solver down:

• Fetch errors and timeouts log a warning and resolve without the remote layer;
  the caller bounds the fetch with a timeout (binary: 2 s)
• Bounded retries with backoff for transient failures only (connect errors, 5xx)
• Response size capped at 256 KiB before parsing
• Non-2xx statuses outside 4xx/5xx are rejected explicitly (S3 answers an
  unfollowable 301 on region mismatches)
• Unknown fields in the payload are ignored (forward compatibility); a payload
  whose pools reference an unknown algorithm is rejected wholesale so outdated
  binaries fall back instead of failing at solver build
• No panic paths reachable from network input
• The final resolved config is validated as a whole, regardless of source

Library API

• Config::apply_remote(url, timeout) — async layer application; returns self
  unchanged on any fetch problem
• config::get_default(chain) — embedded + remote with default URL and built-in
  2 s budget, in one call
• config::remote::fetch_remote_config(url) — raw fetch for custom error handling

Covered by mock-server tests: valid payload, 5xx retry, 4xx no-retry, unfollowable
redirect, oversized body, garbage TOML, unknown algorithm, and fallback-on-timeout.

🤖 Generated with Claude Code

[brunoguerios]

Nice one!
Fits in nicely on top of the previous PR 👏
LGTM ✅