Refine feed planning and recovery strategy

[ehwan]

• Add deterministic pre-feed planning so feed_location simulates CFG-level reductions/shifts first and only mutates stacks after a valid terminal shift is confirmed.
• Reuse the same deterministic planning path for can_feed, can_accept, can_panic, and synthetic error token recovery.
• Restrict recovery to true NoAction cases: GLR recovery now runs only when every active branch is grammatically unable to consume the terminal.
• Change GLR feed success to return FeedSuccess, allowing successful feeds to report errors from pruned sibling branches.
• Remove NoPrecedence, remove GLR state-0 error recovery, and shrink internal GLR failure payloads to avoid unnecessary cloning.

[gemini-code-assist]

Code Review

This pull request refactors the token feeding mechanism in both deterministic and GLR parsers to use a pre-feed simulation phase. In deterministic parsing, this prevents partial stack mutations on grammatical failures. In GLR parsing, feeding now returns a FeedSuccess struct containing optional pruned-branch errors, and panic-mode recovery is only entered when all branches fail with NoAction. The review feedback suggests optimizing performance in the GLR branch loop by hoisting the extra_state_stack allocation out of the loop, and adding a defensive check in the deterministic pre-feed simulation to prevent potential integer underflow.

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[gemini-code-assist]

Allocating a new Vec for extra_state_stack inside the current_branches loop can be inefficient, especially when there are many active branches or during frequent token feeding. Moving the allocation outside of the loop and clearing it on each iteration avoids unnecessary reallocations.

        let mut feedable_branch_seen = false;
        let mut extra_state_stack = Vec::new();
        for Branch { node, userdata } in current_branches.drain(..) {
            // Classify this branch before mutating the graph-structured stack. Only branches that
            // fail this CFG simulation are considered `NoAction` branches.
            extra_state_stack.clear();

[gemini-code-assist]

To prevent potential integer underflow panics in debug mode (or wrapping in release mode) when left > stack_len, add a defensive check to ensure left <= stack_len before performing the subtraction.

                if tokens_len <= extra_state_stack.len() {
                    extra_state_stack.truncate(extra_state_stack.len() - tokens_len);
                } else {
                    let left = tokens_len - extra_state_stack.len();
                    extra_state_stack.clear();
                    if left > stack_len {
                        return Err(FeedPlanError {
                            state: self.simulated_state(&extra_state_stack, stack_len),
                        });
                    }
                    stack_len -= left;
                }