feat(config): introduce tuning presets (balanced, deep)

[zizou0x]

Stacked on #328 (remote config layer).

What

Adds the preset concept: a config::Preset (balanced | deep) selects which
defaults apply. Both the embedded default_config.toml and the per-chain remote
document hold one section per preset (same layout in both files); every layer above
— local config file, CLI flags, legacy worker_pools.toml — behaves unchanged on top.
The two presets currently share identical values; deep gets tuned later.

• Selection: --preset / PRESET (default balanced); lib: get_default(chain, preset),
  Config::apply_remote(url, preset, timeout). A preset can never be set from a config file.
• Remote URL stays one document per chain (presets/{chain}/latest.toml); the fetch
  selects the preset's section. A missing section (or empty document) is a quiet
  "nothing tuned" state — the already-published empty payloads remain valid.
• The unknown-algorithm safety check runs only on the selected section, so a future
  preset section can use newer algorithms without breaking binaries on other presets.
• Preset is strum-derived; parsing, listing, and --help values are generated, and a
  unit test fails if the enum and the file sections drift. Adding a preset = one
  variant + one section per file.
• FyndBuilder::new and fynd-rpc's builtin_default() pin to balanced.

From the design review (fresh-context agent)

Applied: unknown keys in hand-written config files now warn (remote stays fully
permissive for forward compatibility, guarded by a field-list sync test); the remote
layer logs exactly which fields it overrides; the standalone router-timeout exception
is documented at the constant; layer-count doc fix; --help text cleanup.

Not applied: clap::ValueEnum for Preset (fynd-core has no clap dependency — strum
is the boundary-respecting choice); trimming get_default (deliberate convenience
API); changing legacy worker_pools.toml pools precedence (explicit
backward-compatibility decision, already warns); value-range floors in validate()
(no tuning bounds established yet — the new override logging provides visibility
instead).

🤖 Generated with Claude Code

[brunoguerios]

LGTM ✅