Use exact monetary arithmetic and preserve millisatoshi precision

[gcomte]

Conversions previously used f64, losing decimal precision before rounding. For example, converting 1,500 MSAT to BTC produced 0.00000001 BTC. Preserve decimal values from CLI input, legacy YAML defaults, and ticker JSON, calculate exact rate ratios, and round only the final result with midpoint ties away from zero.

Bitcoin denominations now retain millisatoshi precision: BTC has up to 11 decimals, MBTC 8, BITS 5, SAT 3, and MSAT 0. For example, 1 MSAT becomes 0.00000000001 BTC, and 1,500 MSAT becomes 0.000000015 BTC. --integer continues to round to whole output units, while fiat output keeps its existing currency precision.

Bound numeric inputs and exponents, validate positive rates, and report missing rates or conversion overflow without partial stdout. The ticker response is bounded before arbitrary-precision parsing. YAML reading uses the existing parser backend directly to retain numeric text; existing numeric defaults remain compatible.

Validation:

• 33 deterministic tests pass: 26 unit/property tests, 3 exact-conversion CLI tests, and 4 input-validation tests.
• Coverage includes full-u64 millisatoshi round trips, signed midpoint boundaries, values above 2^53, exact YAML/JSON values, repeating fiat cross-rates, invalid rates, and overflow errors.
• cargo fmt --all --check and cargo clippy --all-targets -- -D warnings pass.
• Post-implementation security review found no actionable findings.
• Existing live API tests remain enabled in GitHub CI.

Stacked on #542; review this diff against fix/cli-input-validation. Merge the input-validation PR first and retarget this PR to master if GitHub does not do so automatically.

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