feat(config): centralize solver defaults into a layered config file

[zizou0x]

What

All solver-tuning defaults now live in one file,
fynd-core/src/config/default_config.toml, embedded in the binary. It
deserializes directly into a complete config::Config (every field
required except the chain-specific min_tvl), so adding a config field
without updating the file fails the build's tests.

Configuration resolves field by field through three layers, highest
priority first:

1. CLI flags (only flags the user actually set)
2. Local config file: --config-file / CONFIG_FILE, default ./fynd.toml
3. Embedded default config

Library API

• config::embedded_default() returns the cached, complete default Config
• Config::apply(&PartialConfig) overlays a layer (exhaustive
  destructuring: a new PartialConfig field without an overlay arm is a
  compile error)
• Config::validate() range-checks the resolved result;
  FyndBuilder::apply_config(&Config) validates automatically
• FyndBuilder::new seeds its defaults from the embedded config

Backward compatibility

• worker_pools.toml (file or --worker-pools-config/WORKER_POOLS_CONFIG)
  keeps working; its pools override the config file's so existing
  deployments keep their pools. A deprecation warning points to fynd.toml.
• The repo's worker_pools.toml is renamed to fynd.toml and extended
  into a full config example; docker-compose and the record-market /
  integration-test includes follow.
• Breaking for lib consumers: the tuning constants in
  fynd_core::solver::defaults (and their fynd-rpc re-exports) are
  removed; read config::embedded_default() instead.

Behavior

Defaults are unchanged — the embedded values match the previous
constants, and --help now renders them from the config file. Startup
logs the full resolved config.

🤖 Generated with Claude Code

[github-actions]

Breaking API Changes (Intentional)

Breaking API changes detected and declared in the PR title.
Ensure the minor version is bumped before merging (breaking changes on 0.x.x bump the minor).

semver-checks output

Checking fynd-core v0.90.2 -> v0.90.2 (no change; assume minor)
     Checked [   1.054s] 196 checks: 194 pass, 2 fail, 0 warn, 49 skip

--- failure auto_trait_impl_removed: auto trait no longer implemented ---

Description:
A public type has stopped implementing one or more auto traits. This can break downstream code that depends on the traits being implemented.
        ref: https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/auto_trait_impl_removed.ron

Failed in:
  type SolverBuildError is no longer UnwindSafe, in fynd-core/src/solver.rs:255
  type SolverBuildError is no longer RefUnwindSafe, in fynd-core/src/solver.rs:255
  type SolverBuildError is no longer UnwindSafe, in fynd-core/src/solver.rs:255
  type SolverBuildError is no longer RefUnwindSafe, in fynd-core/src/solver.rs:255

--- failure pub_module_level_const_missing: pub module-level const is missing ---

Description:
A public const is missing or renamed
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/pub_module_level_const_missing.ron

Failed in:
  MIN_TOKEN_QUALITY in file fynd-core/src/solver.rs:63
  GAS_REFRESH_INTERVAL in file fynd-core/src/solver.rs:69
  ROUTER_MIN_RESPONSES in file fynd-core/src/solver.rs:76
  TRADED_N_DAYS_AGO in file fynd-core/src/solver.rs:65
  TVL_BUFFER_RATIO in file fynd-core/src/solver.rs:67
  RECONNECT_DELAY in file fynd-core/src/solver.rs:73

     Summary semver requires new major version: 2 major and 0 minor checks failed
    Finished [   1.444s] fynd-core
    Checking fynd-test-fixtures v0.90.2 -> v0.90.2 (no change; assume minor)
     Checked [   0.119s] 196 checks: 196 pass, 49 skip
     Summary no semver update required
    Finished [   0.181s] fynd-test-fixtures
    Checking fynd-rpc v0.90.2 -> v0.90.2 (no change; assume minor)
     Checked [   0.112s] 196 checks: 196 pass, 49 skip
     Summary no semver update required
    Finished [   0.173s] fynd-rpc
    Checking fynd-rpc-types v0.90.2 -> v0.90.2 (no change; assume minor)
     Checked [   0.193s] 196 checks: 196 pass, 49 skip
     Summary no semver update required
    Finished [   0.292s] fynd-rpc-types
    Checking fynd-client v0.90.2 -> v0.90.2 (no change; assume minor)
     Checked [   0.240s] 196 checks: 196 pass, 49 skip
     Summary no semver update required
    Finished [   0.376s] fynd-client
    Checking fynd-tools-common v0.90.2 -> v0.90.2 (no change; assume minor)
     Checked [   0.091s] 196 checks: 196 pass, 49 skip
     Summary no semver update required
    Finished [   0.143s] fynd-tools-common
    Checking erc20-overrides v0.90.2 -> v0.90.2 (no change; assume minor)
     Checked [   0.092s] 196 checks: 196 pass, 49 skip
     Summary no semver update required
    Finished [   0.132s] erc20-overrides

[brunoguerios]

Elegant solution! 😃
LGTM ✅

[brunoguerios]

Are these inline imports intentional?
Should we move them to the top of the file?

[brunoguerios]

Nice cleanup/improvement 👏

[brunoguerios]

Nice flow 👏