fix(tdbe): reject non-positive spot/strike and size u8 layout fields correctly

[userFRM]

Two numeric-safety fixes in the core crate

Non-positive spot/strike no longer emit NaN or null Greeks

Black-Scholes is undefined for spot <= 0 or strike <= 0: the (spot / strike).ln() term goes non-finite and strike == 0 is an outright divide-by-zero. The existing degeneracy guard only checked vol and time, so a non-positive spot or strike flowed straight into d1/d2, and the fields that did not route through the NaN-to-zero clamp serialised to JSON null.

• The fallible public entry points all_greeks and implied_volatility (the surface every SDK / CLI / MCP funnels through) now reject a non-positive or non-finite spot/strike up front with Error::Config, naming the offending value.
• The infallible compute_full_bundle_with_iv treats the same inputs as degenerate and returns an all-zero bundle rather than producing NaN/Inf.

The positivity test is written as value > 0.0, so a NaN spot or strike also routes to the rejection / degenerate path.

u8 sized correctly in the FPSS layout-assert generator

The C-layout generator gave u8 a size and alignment of 4 by sharing the i32 match arm. That is harmless today only because the single u8 field is followed by an 8-aligned field, but any future u8-before-smaller-aligned field would have baked a wrong offset into a generated static_assert. The arm is split so u8 is 1/1 and i32 is 4/4. The other arms each map one logical width, so no further conflation remained to fix.

Test evidence

cargo test -p thetadatadx --lib tdbe::greeks — 26 passed, 0 failed. New tests:

• all_greeks_errors_on_nonpositive_spot
• all_greeks_errors_on_nonpositive_strike
• implied_volatility_errors_on_nonpositive_spot
• implied_volatility_errors_on_nonpositive_strike
• compute_full_bundle_zeros_on_nonpositive_spot_or_strike

Each checks spot/strike of 0.0 and negatives return a clean Error::Config (no NaN, no panic), and that the infallible bundle path returns an all-zero, all-finite bundle.

Also verified: cargo fmt --all -- --check clean, cargo build -p thetadatadx succeeds (exercises the layout generator's static_assert emission via build.rs), cargo clippy -p thetadatadx --lib -- -D warnings clean.

🤖 Generated with Claude Code