Add classic acceleration exponent

[SimoneMariaRomeo]

Summary

• add a CLASSIC_EXPONENT kernel parameter for the existing linear/classic curve
• expose it through the Rust CLI/TUI parameter list and NixOS module as classic-exponent / classicExponent
• preserve current behavior by defaulting to exponent 2, while allowing values above 2 for a more classic-style curve

Validation

• git diff --check
• not run: cargo fmt, cargo test, driver tests, or kernel-module build because this Windows environment has no Cargo/Rust/C toolchain and Docker is not running

Fixes #111.

Summary by CodeRabbit

Release Notes

• New Features
  • Added a new classicExponent parameter for the linear acceleration curve. Default value is 2, ensuring backward compatibility. Exponent values greater than 2 adjust the curve shape near the acceleration offset and at higher input speeds.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 54c0f922-10b0-4d84-a8db-edb2e3b9f633

📥 Commits

Reviewing files that changed from the base of the PR and between c7c1369 and 070fb51.

📒 Files selected for processing (11)

• README.md
• README_NIXOS.md
• crates/core/src/context.rs
• crates/core/src/params.rs
• crates/core/src/persist.rs
• crates/core/src/sens_fns.rs
• driver/accel/linear.h
• driver/accel_k.h
• driver/params.h
• driver/tests/accel.test.c
• module.nix

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📝 Walkthrough

Walkthrough

This PR adds a configurable classic_exponent parameter to the linear acceleration model, enabling classic-style curves where exponent values greater than 2 produce steeper sensitivity curves. Changes span the Rust parameter system, kernel driver implementation, NixOS configuration, and test coverage with validation and documentation updates.

Changes

Classic Exponent Implementation

Layer / File(s)	Summary
Rust parameter definition crates/core/src/params.rs	Added ClassicExponent enum variant to Linear mode parameters, mapped to kernel name "CLASSIC_EXPONENT", display name "Classic Exponent", with description noting that exponent 2 preserves existing behavior; validation rejects non-positive values.
Rust parameter propagation crates/core/src/context.rs, crates/core/src/persist.rs, crates/core/src/sens_fns.rs	Parameter is captured in params_snapshot, persisted to sysfs via set_all_linear, and forwarded to the sensitivity function's acceleration calculation pipeline.
Kernel linear acceleration implementation driver/accel/linear.h	linear_curve_args struct gains classic_exponent field; linear_base_fn accepts exponent and computes power using fast-path square for exponent 2 or general fpt_pow; __linear_sens_fun logs exponent and clamps non-positive values to 2 before passing to linear_base_fn.
Kernel module parameter wiring driver/params.h, driver/accel_k.h	Module parameter CLASSIC_EXPONENT defined with architecture-dependent fixed-point defaults; collect_args initializes the parameter from PARAM_CLASSIC_EXPONENT when using linear mode.
NixOS configuration module.nix	Added hardware.maccel.parameters.classicExponent option (nullable float, > 0.0); wired to kernel module parameter string generation via fixed-point encoding.
Tests and documentation driver/tests/accel.test.c, README.md, README_NIXOS.md	Linear test setup extended to pass exponent value; new unit test test_classic_exponent_changes_linear_curve asserts that exponent 3 produces higher sensitivity than exponent 2; parameter documented in README with default and qualitative curve effects; NixOS Quick Start example includes classicExponent = 2.0;.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

🐰 A hop through acceleration's might,
Where exponents dance and curves take flight,
From Rust to kernel, the parameters flow,
Classic style now has a dazzling show,
Configuration tames the power below!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 23.08% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change—adding a classic acceleration exponent parameter to the linear acceleration curve.
Linked Issues check	✅ Passed	The PR fully implements the objective from issue #111 by adding a configurable exponent parameter to the linear acceleration formula with a default value of 2.
Out of Scope Changes check	✅ Passed	All changes are directly related to implementing the classic acceleration exponent feature; no out-of-scope modifications were detected.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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