Fix ToInt64RoundZero edge cases, add Sin/Cos/Tan and tests, update README

[jenska]

Motivation

• Correct incorrect behavior and sign/overflow handling in the ToInt64RoundZero conversion for edge cases and shifting semantics.
• Provide basic trigonometric helpers (Sin, Cos, Tan) for the X80 type to broaden the math API.
• Add unit tests to cover the new conversion behavior and trig functions, and update documentation to advertise the new APIs.

Description

• Fixed ToInt64RoundZero in conversions.go by reordering the overflow/edge-case branch, returning math.MinInt64 for the exact negative-int64 boundary, adjusting the shift to avoid sign-extension (aSig >> (-shiftCount)), and ensuring exceptions are raised correctly.
• Added Sin, Cos, and Tan methods to operations.go that return NaN for NaN inputs and otherwise call the standard library via math.Sin, math.Cos, and math.Tan with conversions to/from float64.
• Imported math where required and updated tests: added TestX80_ToInt64RoundZero and TestX80_SinCosTan in conversions_test.go and operations_test.go respectively to validate behavior and NaN/Infinity handling.
• Updated readme.md to document the new trig methods and the round-toward-zero conversion variants (ToInt32RoundZero, ToInt64RoundZero).

Testing

• Ran unit tests with go test ./... covering conversions and operations including TestX80_ToInt64RoundZero and TestX80_SinCosTan.
• All automated tests completed successfully.

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Codex Task