bug: `ShapeTable.registerWithId` race / `next_id` wraparound enables shape hijack

[minagishl]

Bug Description

ShapeTable.registerWithId (src/session.zig:124-140) and SessionEncoder.encodeWithSchema (:1521-1527) overwrite or accept inconsistent state under adversarial conditions:

pub fn registerWithId(self: *ShapeTable, allocator: Allocator, shape_id: u64, keys: []const []const u8) !bool {
    if (self.by_id.get(shape_id)) |existing| {
        return eqlNestedBytes(existing.keys, keys);
    }
    if (try self.getId(allocator, keys)) |existing_id| {
        if (existing_id != shape_id) {
            return false;
        }
    }
    try self.insertShape(allocator, shape_id, keys);
    if (self.next_id <= shape_id) {
        self.next_id = shape_id + 1;
    }
    return true;
}

Three issues:

1. If the attacker can inject a RegisterShape before the legitimate sender's, the attacker's shape becomes canonical. Subsequent ShapedObject(shape_id = X) decodes are read with the attacker's key list.
2. next_id = shape_id + 1 can be set to u64-near-max by the attacker. Subsequent next_id +%= 1 (wrapping) collisions with previously-registered ids.
3. The two directions of "same shape, different id" are checked asymmetrically.

Steps to Reproduce

1. Attacker registers shape id=0 with keys=["admin","role"].
2. Legit sender later registers shape id=0 with keys=["role","user"] → registerWithId returns false (existing matches keys), but legit sender's subsequent ShapedObject(0, [string1, string2]) is decoded with the attacker's keys.

Expected Behavior

1. Reject registerWithId when the same keys already map to a different id (already partially checked at :131, but the inverse direction should also fail).
2. Require next_id updates to use std.math.add(u64, ..., 1) and return RecurramError.InvalidData on overflow.
3. Document and harden the trust model for shape registration (e.g., per-peer namespacing).

Actual Behavior

Race-condition hijack possible; wrap-around possible.

Environment

• Project: recurram-zig
• File: src/session.zig:124-140

Additional Context

Severity: High — state corruption attack on cooperating session.