Cross-reference FLOAT_E against e derived in Rust, read through e()

[thedavidmeister]

Adds Float::e(), which calls the concrete's e() and so returns LibDecimalFloat.FLOAT_E from the compiled library, and a Rust cross-reference of that value: e derived by the 1/k! series in 512-bit integer arithmetic with 12 guard digits, rounded to nearest at 66 places, asserted equal to the getter's return both by pack_lossless of the derived coefficient and by Float::parse of the derived digits through the contract's parser. No literal is mirrored in Rust; what is checked is the library's Solidity, reached through the concrete.

FLOAT_PI (rain.math.float#274) takes the same path in a follow-up once that release is pinned here and the concrete exposes pi() beside e(). That getter moves DecimalFloat's bytecode, so it is its own change.

QA

• Discriminating tests: e_is_e_rounded_to_nearest and e_parses_from_its_digits; a one-ulp change to FLOAT_E in the library fails both, and the series is independent of the library's digits.
• Mutations applied: n/a, a getter wrapper and tests.
• Oracle: the exponential series evaluated in integers; no floating point and no copied digits.
• Category check: the ask was a Rust cross-reference bound to the Solidity constants; this is the binding for e, and the pattern π follows.

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https://claude.ai/code/session_01QyCCzi9WZPhuXcU1hwr2bq

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Reviewing files that changed from the base of the PR and between 8f1f4ca and ff22fea.

📒 Files selected for processing (2)

• crates/float/src/constants.rs
• crates/float/src/lib.rs

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[thedavidmeister]

Withdrawn: the cross-reference belongs in rain.math.float beside the constants; it lands there on rain.math.float#274.