feat(config): optional TOML config loading behind config-file feature

A file-based way to set tuning knobs without recompiling. urus is a
library, so it never presumes a config path or reads the environment — the
binary hands the text in:

- Config::with_toml_str(&str) -> Result<Config, ConfigError>: sparse overlay
  onto an existing Config (built with the addr the binary chose). Only keys
  present are applied; durations are integer seconds; unknown keys are a
  hard error (deny_unknown_fields) so a typo is loud, not a silent no-op.
- Scope: the four slowloris knobs (head_timeout_secs, body_timeout_secs,
  body_burst_bytes, body_stall_timeout_secs). Migrating the rest of Config
  into the file is a separate, additive job — TomlOverrides just grows.
- Feature `config-file = ["dep:serde", "dep:toml"]`; the optional serde dep
  gains the derive feature. The default build is unchanged (deps + code are
  all gated).
- examples/serve_toml.rs (required-features = ["config-file"]): a `--config
  PATH` demo with no presumed default location. plain_serve and its env
  vars are left untouched.

Tests (feature-gated): empty keeps defaults, partial overrides only named,
full overrides all, unknown key errors, malformed errors. 84 lib with the
feature / 79 without; clippy --lib clean both ways; e2e smoke serves 200
from a file and rejects an unknown key loudly.

Cargo.toml

@@ -14,14 +14,17 @@ sha1_smol = "1"
 
 # dep #4, ratified 2026-06-12: serde/serde_json behind the opt-in
 # "phoenix" feature only — the "channels" core stays dependency-free.
-serde      = { version = "1", optional = true }
+serde      = { version = "1", optional = true, features = ["derive"] }
 serde_json = { version = "1", optional = true }
+# config-file feature: TOML loader for tuning knobs (dep #5, 2026-08-12)
+toml       = { version = "0.8", optional = true }
 
 [features]
 smarm-trace  = ["smarm/smarm-trace"]
 smarm-causal = ["smarm/smarm-causal"]
 channels     = []
 phoenix      = ["channels", "dep:serde", "dep:serde_json"]
+config-file  = ["dep:serde", "dep:toml"]
 
 [dev-dependencies]
 serde      = { version = "1", features = ["derive"] }
@@ -47,3 +50,7 @@ path = "examples/crud.rs"
 name = "channels_chat"
 path = "examples/channels_chat.rs"
 required-features = ["phoenix"]
+
+[[example]]
+name = "serve_toml"
+required-features = ["config-file"]