Initial landing of Core

[SpookyYomo]

As discussed from #53, we begin the migration by first introducing the -core crate.
By using the version number as being 0.0.0, we show strictly that it's meant to be a internal crate.

I believe we can start defining the common traits necessary on non-NAN floats, integers etc here too. I am not sure at present if this conflicts with the internal crate requirement.

Currently just providing a minimal set of Error enums as cherry-picked, with non-allocating variants.
The fmt::Display for the error and a README.md is to be decided in the future.

Edit:
The results as shown in #78 and #86 might show that the current Errors might be strictly insufficient are require more thought. It might be worth seriously considering anyhow.

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Note

Introduce new sci-rs-core crate with no_std-ready core error types, feature gates, and base deps.

• Core crate: Add sci-rs-core with no_std support and feature flags.
  • Features: alloc (default), std (enables alloc).
  • Metadata: docs.rs all-features = true.
  • Deps: ndarray, ndarray-conv, num-traits (minimal default features).
• Error handling:
  • New Error enum with InvalidArg and ConflictArg variants.
    • Allocating variants (with String) under alloc.
    • Non-alloc fallbacks without payloads when alloc is disabled.
  • fmt::Display and core::error::Error impls with conditional messaging.
  • Result<T> type alias to core::result::Result<T, Error>.
• Files:
  • sci-rs-core/Cargo.toml
  • sci-rs-core/src/lib.rs

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[SpookyYomo]

Rebased onto new main

[SpookyYomo]

Perhaps some error conventions and things to consider as this develops into the future: https://sabrinajewson.org/blog/errors