Refactor context.rs

[cedoor]

Summary

Split src/context.rs into focused modules (or submodules) so the crate entry point stays maintainable as the API grows.

Background

context.rs currently holds Params, ContextOptions, Context, scratch sizing (compute_arena_bytes), and related helpers in one file. That was fine at small scope, but milestone 2 adds more bin_fhe surface; a single large file makes reviews harder and increases merge conflict risk.

Work

• Decide module split (e.g. params.rs, options.rs, context.rs re-exports only; or context::params / context::options submodules).
• Keep lib.rs as declarations + re-exports only per conventions.mdc.
• Preserve the public API (Context, Params, ContextOptions paths from the crate root).
• No behavior change intended — move-only refactor unless a small internal cleanup is clearly safe.