fix(ENG-570): Reconcile Price codec forks beyond the FX range

[alnoki]

Makes the Rust and TS Price codecs agree for inputs above the protocol's FX target band (~1e-6 .. 1e6), closing two divergences found in the ENG-557 adversarial review (PR #140). Both live in the shared Price codec, so they land together.

1. Encode huge-value divergence

Price::from_value silently saturated the f64→u64 cast — (1e300 * 1e7) as u64 clamps to u64::MAX, which from_scaled then normalized into a bogus finite Price (~1.8e12). TS encodePrice instead threw RangeError. So the forks rejected different input sets.

Fix: both forks now reject any finite value whose value * 1e7 would overflow u64 (the exact saturation point, ≥ 2^64). Below that boundary both truncate the same f64 toward zero and normalize with identical integer arithmetic, so the reject and accept sets are now bit-identical. This fixes the real bug (Rust's silent normalization) without imposing an arbitrary hard 1e6 cap that would reject structurally-valid prices.

2. weightedAverage fidelity (from ENG-572)

Bit-identical to Rust inside the FX band, but two out-of-band gaps in the TS fork:

• Gap 1 — precision. fromScaled(Number(avg), 14) rounded avg (a bigint up to u64::MAX) through an f64 once avg > 2^53, flipping the 8th significand digit vs Rust's exact u64 from_scaled. fromScaled is now bigint-based and driven from the bigint directly.
• Gap 2 — saturation. Rust uses saturating_mul/saturating_add on the w*v products and their sum; TS used plain bigint. Now clamped at u128::MAX to mirror Rust.

The shared bigint fromScaled is what makes #1 and #2 one change.

Conformance

• price_vectors.json — huge-value rejects (1e13, 1e300) + a large-but-valid accept (1e12) pinning the boundary.
• share_vectors.json — two out-of-band merge_entry_basis cases: a precision case (the bigint path yields significand 60_000_000; the old Number(avg) path produced 60_000_001) and a u128-saturation case.
• docs/interface.md §6B — the "deliberately unconformed" note is replaced with the now-agreed contract.

Verification

• cargo test -p dropset-math-core --tests — 81 unit + 5 price-conformance + 6 share-conformance, all green.
• pnpm --filter @dropset/sdk test — 13 TS conformance tests green (both forks replay the regenerated vectors).
• cargo fmt / cargo clippy clean.

[linear]

ENG-570