Add --radix flag, tracing, strict parsing, and broader test coverage

[devjgm]

This change establishes a strong correctness contract: a clean run with no warnings means every input value was successfully parsed and counted. It introduces a more flexible --radix flag, replaces the old -x shortcut, and migrates internal logging to tracing.

Breaking changes

• Removed -x, --hex flag. Replaced by --radix <auto|hex|decimal> (default: auto). To preserve old behavior, use --radix=hex.
• --radix=hex is strict. With this flag (or a 0x prefix), values that fail to parse as hex no longer silently fall back to float -- they warn and contribute 0.
• New --radix=decimal mode rejects 0x-prefixed values entirely, useful when input may legitimately contain 0x-prefixed strings that are not numbers.
• Integer overflow now panics in release builds, not just debug. Uses checked_add to make the behavior consistent across profiles.

Behavior changes (warnings)

Every silently-skipped value now emits a warning, so users can trust that the absence of warnings means a correct sum:

• Failed parses (any non-numeric value) now warn.
• Field index out of range now warns and skips the line.
• Comma stripping ("1,000" -> "1000") now warns, surfacing potential locale mismatches ("1,5" -> "15" is silent no longer).
• Large integers that overflow i128 and fall back to f64 now warn about possible precision loss.

Internal changes

• Replaced log + env_logger with tracing + tracing-subscriber (structured logging, better field formatting). RUST_LOG continues to work.
• Replaced std::fs with fs-err for clearer file-open error messages (now includes the path).
• Extracted a self-contained parse_value(&str, Radix) function with strict-hex semantics.
• Various readability cleanups: iterator-based reader setup, flattened reader/line loops, let-else for field extraction, simplified verbose output.

Tests

• Added 11 unit tests for parse_value covering decimal/hex, integer/float, negatives, scientific notation, hex strictness, invalid input, empty string, and overflow.
• Added integration tests for: single-file/multi-file input, nonexistent files, field 0, out-of-range field, negative integers/floats/mixed, comma-formatted numbers, invalid 0x prefix, large-integer overflow warning, strict hex mode, --radix=decimal mode, empty input.

Verbose output (-v) changes

• radix= now shows Hex/Decimal instead of 16/10.
• Removed cnt= field (was unused outside verbose output).
• err= now contains the warning message rather than the raw ParseFloatError debug string.

Documentation

• README updated for the new --radix flag, accurate warning examples, and the strict-hex contract.