fix: improve stacksafe macro diagnostics

[tisonkun]

Summary

• reject const fn and duplicate crate = ... arguments with focused diagnostics
• preserve Syn parse errors for malformed functions and improve non-function spans
• disable unused Syn default features and avoid unnecessary cloning and proc_macro token conversion
• document the const fn limitation and add trybuild coverage

Out of scope

• #[track_caller] behavior is intentionally unchanged and will be handled separately

Testing

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- --deny warnings
• cargo test -- --nocapture
• cargo build --workspace --all-targets --release
• cargo +1.85.0 build --workspace --all-targets
• cargo +1.85.0 test -- --nocapture
• taplo check
• typos
• hawkeye check
• cargo release -p stacksafe-macro patch --no-publish --no-push --no-tag --no-confirm -vv

[Copilot]

Pull request overview

Improves the #[stacksafe] proc-macro’s error reporting and diagnostics, adds UI coverage for the new diagnostics, and tightens syn dependency configuration to reduce unnecessary features while keeping required parsing/printing support.

Changes:

• Adds targeted diagnostics for const fn usage and duplicate crate = ... parameters.
• Preserves syn parse errors for malformed functions and improves spans for non-function items.
• Adds trybuild UI cases for malformed functions, duplicate parameters, and const functions; updates public docs to note the const fn limitation.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
stacksafe/tests/ui/non_function_item.stderr	Updates expected UI output to reflect improved non-function span/labeling.
stacksafe/tests/ui/malformed_function.stderr	New expected UI output asserting preserved syn parsing errors.
stacksafe/tests/ui/malformed_function.rs	New UI test case for malformed function parsing.
stacksafe/tests/ui/duplicate_parameter.stderr	New expected UI output for duplicate crate parameter diagnostics.
stacksafe/tests/ui/duplicate_parameter.rs	New UI test case for duplicate crate parameter handling.
stacksafe/tests/ui/const_function.stderr	New expected UI output for rejecting const fn.
stacksafe/tests/ui/const_function.rs	New UI test case for const fn rejection.
stacksafe/src/lib.rs	Documents const fn as a limitation of #[stacksafe].
stacksafe-macro/src/lib.rs	Implements improved parsing/diagnostics (duplicate params, const rejection, better non-function spans, preserved parse errors).
stacksafe-macro/Cargo.toml	Enables only required syn features for macro parsing/printing.
Cargo.toml	Disables syn default features at the workspace level.

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