fix: target-pointer-width must be an integer in target spec JSON

[sagar-a16z]

rustc 1.94 tightened JSON schema validation and rejects the string form.

    --- stderr                                                                                                                                                                                                                                                                
    error: error loading target specification: target-pointer-width: invalid type: string "64", expected u16 at line 9 column 30                                                                                                                                              
      |                                                                                                                                                                                                                                                                       
      = help: run `rustc --print target-list` for a list of built-in targets`    

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PR Summary

Medium Risk
Updates the pinned Rust toolchain and changes target spec generation based on rustc version, which can affect build outputs and CI environments if version detection fails or differs across toolchains.

Overview
Updates target spec generation to satisfy newer rustc JSON validation by rendering target-pointer-width as a JSON integer (and conditionally falling back to the old quoted string form for rustc < 1.91 via a rustc --version check).

Bumps the pinned toolchain to 1.94, and adjusts the RISC-V ARCH_OPS ret_from_fork export to cast the function to usize via *const () for compatibility with newer Rust casting rules.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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Version threshold for pointer-width format may be wrong

Low Severity

The code comment claims the target-pointer-width JSON type change happened in rustc 1.91, but the PR description states "rustc 1.94 tightened JSON schema validation and rejects the string form." The threshold minor < 91 may need to be minor < 94. If the change actually landed in 1.94, rustc versions 1.91–1.93 would receive the integer form when they still expect a string, producing invalid target JSON for anyone using those compiler versions.