Add TOML config file support with layered CLI overrides and validation

[dcorral]

Adds support for configuring the node via a TOML file, loaded from <storage_directory_path>/config.toml or the path given with --config. Values are resolved in order: built-in defaults, then the config file, then explicit CLI options. All current defaults are unchanged.

This moves the hardcoded policy constants (fee rate, UTXO size/count, channel confirmations, HTLC minimum, channel capacity bounds, swap fee cap, CLTV deltas, timers) and the LDK-inherited channel knobs (cltv_expiry_delta, to_self_delay, forwarding fees, dust exposure, in-flight limits, accept_forwards_to_priv_channels) into a [section]-based config, so nodes can be tuned without recompiling. The existing CLI args are also readable from the file, with the CLI taking precedence.

Every value is validated at startup (errors name the offending key) and unknown keys are rejected. All keys are documented with their defaults in sample-config.toml.

[codecov]

Codecov Report

❌ Patch coverage is 94.02985% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.54%. Comparing base (6887cbf) to head (46193a8).

Files with missing lines	Patch %	Lines
src/args.rs	86.50%	22 Missing ⚠️
src/routes.rs	88.00%	9 Missing ⚠️
src/config/file.rs	66.66%	4 Missing ⚠️
src/config/mod.rs	99.67%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
+ Coverage   87.78%   88.54%   +0.75%     
==========================================
  Files          16       18       +2     
  Lines        8459     9000     +541     
==========================================
+ Hits         7426     7969     +543     
+ Misses       1033     1031       -2     

Flag	Coverage Δ
rust	88.54% <94.02%> (+0.75%)	⬆️

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