[codex] Switch configuration system to TOML

[Bathtor]

Summary

• switch Kompact's configuration loading from HOCON to TOML
• replace the parser-exposed config tree with Kompact-owned Config and ConfigValue types
• add a path-aware fallible lookup API while keeping indexing as a panicking convenience API
• update examples and docs to explicit TOML filenames and TOML syntax

Motivation

Rust support for HOCON is not in a good state, while TOML is well-supported and already familiar to Rust users. Since Kompact is still pre-1.0, this change intentionally takes the breaking API clean-up now instead of carrying parser-specific design constraints forward.

API Changes

• KompactSystem::config() and ComponentContext::config() now return &Config
• Config is now the public top-level document type; ConfigValue represents a single parsed value
• config["a"]["b"] still works, but it is now a panicking convenience API, just like standard Rust indexing
• fallible access is now config.get("a") for a top-level key and config.select("a.b") for dotted paths; both return a ConfigLookup that keeps the full accessed path for better error messages
• typed config-entry reads are now config.read(&ENTRY) and config.read_or_default(&ENTRY) instead of get and get_or_default
• durations and byte sizes remain available through as_duration() and as_bytes(), and TOML datetimes are preserved via as_datetime()
• set_config_value(...) now injects directly into the overlay config instead of relying on parser-specific string snippets

Config Semantics

• multiple config sources are merged hierarchically: nested tables merge recursively and later sources win
• when the later source changes the value kind, it replaces the earlier value entirely, matching the permissive behaviour we had before with HOCON
• TOML syntax replaces HOCON syntax, so values with units are written as strings such as "100 ms" and "128KiB"

Files And Examples

• the user-facing example file is now app_settings.toml
• the test fixture in core/src/lib.rs is now test_settings.toml
• the local configuration guide now documents the difference between panicking indexing and the new fallible lookup API

Validation

• cargo fmt --all
• cargo test --workspace --no-run
• cargo test --workspace

Fixes #187

[Bathtor]

I think overall we stayed very close to HOCON API, which admittedly reduces breaking changes, but I think we there are places where we can and simply should do better and accept a bit more breakage at this point. It'll only be more painful later.

[Bathtor]

Few more comments, then it's ready.

[Bathtor]

Couple new comments.